High performance multilayer electrodes for use in reducing gases

ABSTRACT

Electrode materials systems for planar solid oxide fuel cells with high electrochemical performance including anode materials that provide exceptional long-term durability when used in reducing gases and cathode materials that provide exceptional long-term durability when used in oxygen-containing gases. The anode materials may comprise a cermet in which the metal component is a cobalt-nickel alloy. These anode materials provide exceptional long-term durability when used in reducing gases, e.g., in SOFCs with sulfur contaminated fuels. The cermet also may comprise a mixed-conducting ceria-based electrolyte material. The anode may have a bi-layer structure. A cerium oxide-based interfacial layer with mixed electronic and ionic conduction may be provided at the electrolyte/anode interface.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.12/001,062, filed on Dec. 7, 2007, entitled “High Performance MultilayerElectrodes for Use in Reducing Gases.” The entire disclosure of theforegoing patent application is incorporated by reference herein.

FIELD OF THE INVENTION

This invention relates to materials and fabrication processes for planarelectrochemical cells. The disclosed materials and processes areparticularly well suited to applications in which high long-termstability, high efficiency operation, sulfur tolerance and/or highvolumetric and gravimetric power densities of stacks of planar cells aredesirable. This invention may be useful in electrochemical gasseparation systems and catalytic reactors, including but not limited tosolid oxide fuel cells for producing power from hydrocarbons andhydrogen-containing fuels, solid oxide electrolysis systems forproducing hydrogen or other fuels from steam or other feedstocks, andceramic oxygen generation systems for producing pure oxygen from air oranother oxygen-containing gas.

BACKGROUND OF THE INVENTION

Demand for efficient and reliable electrical power is escalating,outpacing the improvements in conventional power sources. Applicationsin which compact, lightweight, energy-dense power supplies would findimmediate application include portable power generators, combined heatand power systems, and auxiliary power units for vehicles. Concernsabout global climate change are increasing with the level of CO₂ in ouratmosphere caused by the use of combustion based methods to generatepower from fossil fuels. Fuel cells offer a viable approach to increaseefficiency of power generation from fossil fuels while greatly reducingemissions of pollutants and greenhouse gases. Of the various types offuel cells, the proton exchange membrane (PEM) fuel cell, which operateswith hydrogen as a fuel, has received considerable attention due to itslow weight, low-temperature operation, and ease of manufacture. However,the operation of PEM fuel cells with fossil-based hydrocarbon fuelsrequires extensive pre-processing (reforming) to convert thehydrocarbons into a hydrogen rich gas and subsequent gas purificationsteps to reduce carbon monoxide and sulfur to very low levels (CO<10 ppmand H₂S<10 ppb). Solid oxide fuel cells (SOFCs), which operate at hightemperature (typically, 600 to 1000° C.), are much less sensitive toimpurities in hydrocarbon fuels, which minimizes the amount of gaspurification steps required. This greatly increases power generationefficiency and reduces system complexity. It also is theoreticallypossible to operate solid oxide fuel cells directly on certainhydrocarbon fuels (e.g., methane, methanol and ethanol) via internalreforming, i.e., without an initial reforming step.

A solid oxide fuel cell is comprised of an oxygen ion conducting ceramicelectrolyte membrane that is sandwiched by a fuel electrode (anode) andan air electrode (cathode). Power is generated by passing air (oroxygen) over the cathode and fuel (e.g., hydrogen plus carbon monoxide)over the anode and collecting the electrical current that is created bythe electrochemical reaction of oxygen with fuel to form steam andcarbon dioxide. The key to successful development of SOFC systems is theelectrochemical cell design and “stacking” configuration (i.e., themanner in which SOFC elements are configured within a power producingSOFC module). In this regard, it is important to pack a large amount ofactive area for electrochemical reactions within the smallest volumepossible. A typical solid oxide fuel cell will generate about 30 to 40watts of power for every 100 cm² of active area—this translates to morethan about 3000 cm² of active area for a kilowatt of power. Another keyis maximizing the electrical efficiency of power generation (defined asthe output power divided by the lower heating value of the input fuel).A reasonable target for commercially viable systems is electricalefficiency of greater than approximately 50 percent. This requires thatmost of the fuel fed to the SOFC be used to generate power. The variouscell and stack design alternatives are discussed in the followingparagraphs.

Tubular solid oxide fuel cells include a multi-layer tube fabricatedwith cathode, electrolyte, anode layers, and in some cases interconnectlayers. Tubular SOFCs may be supported by anode, cathode, or electrolytematerials or a porous inert and electrically insulating material withsubsequently deposited thin-film anode, electrolyte, cathode andinterconnect layers. Conventional tubular cells typically suffer fromlow volumetric or gravimetric power density because large tubes do notpack well and have a low surface area to volume ratio. Power densitiesachievable with conventional tubular cells also are limited by the longcurrent collection paths intrinsic to long-length tubular cells.Microtubular SOFCs, typically with diameters of less than about 5 mm,overcome some of the disadvantages of conventional tubular fuel cells.Sealing of small diameter microtubes is simpler than sealing ofconventional tubes. Microtubular cells also overcome the low surfacearea to volume ratio associated with conventional tubular cells.However, microtubular cells require complex manifolding and electricalinterconnection schemes so that scaling to higher powers (more thanabout 100 watts) is difficult.

Planar SOFCs may be supported by either the anode material or theelectrolyte material, also have been demonstrated extensively.Anode-supported cells often are preferred because these cells canaccommodate a thin electrolyte layer (less than about 20 microns). Thisreduces electrolyte ohmic resistance in the cell and allows operation atrelatively low temperatures (e.g., T<800° C.). Planar anode-supportedcells are particularly attractive for mass market, cost-drivenapplications because of their high areal power density and theiradvantageous packing efficiency. Performance of anode-supported cells at700° C. has been demonstrated to be over 1 W/cm² in small cells at lowfuel use. With appropriate seal and interconnect technology, powerdensities greater than 0.4 W/cm² have been reported for anode-supportedcell stacks. However, anode-supported cells are not without drawbacks.When conventional nickel oxide/yttrium-stabilized zirconia (NiO/YSZ)composites are used as support materials, the reduction of NiO to nickelmetal creates stress in the electrolyte layer, which may result inconsiderable deformation of the support. Operating planaranode-supported cells at high power density and high fuel use also isdifficult; the thick porous layer prevents rapid diffusion of steam awayfrom the electrolyte and results in increased cell area-specificresistance (ASR) at high current density.

Electrolyte-supported planar cells have an electrolyte layer thatprovides the mechanical strength of the cell. Electrolyte-supportedcells offer numerous advantages in the production of SOFCs. The sealingof electrolyte-supported cells is simpler than electrode-supportedplanar cells because a dense electrolyte perimeter can be preservedduring cell processing, which provides a dense, smooth surface forsealing operations. Electrolyte-supported cells also have good stabilityduring reduction. Because only a thin layer of anode is affected by thereduction process, this process generally has little impact on cellmechanical stability. The gas diffusion path in and out of the thinneranode layer is short, making fuel and steam diffusion limitations lessof a concern. However, under identical operating conditions,conventional electrolyte-supported cells often exhibit much higherarea-specific resistance values than anode-supported cells because theelectrolyte is more resistive than the anode or cathode materials. Tocompensate for this higher area-specific resistance, the operatingtemperature for electrolyte-supported cells generally is higher thananode-supported cells using the same materials set. The higher operatingtemperature of the electrolyte-supported cells can be a drawback,particularly for developers wishing to use metallic interconnectmaterials.

Two recent U.S. patents, U.S. Pat. No. 8,192,888 (issued Jun. 5, 2012)and U.S. Pat. No. 7,736,787 (issued Ser. No. 11/220,361 (issued Jun. 15,2010), owned by NexTech Materials, Ltd., describe novel planar cellstructures that overcome technical barriers associated with buildingSOFC stacks with conventional planar cells. The first of these, referredto as the FlexCell, comprises a thin electrolyte membrane layer that ismechanically supported by a “honeycomb” mesh layer of electrolytematerial (see FIGS. 1 and 2). More than 75 percent of the electrolytemembrane within the active area of the FlexCell is thin (20-40 microns)and the periphery of the cell is dense. Electrode (anode and cathode)layers are subsequently deposited onto the major faces within the activecell regions to complete the fabrication of an SOFC based on theFlexCell structure. The second structure, referred to as the HybridCell,comprises an anode layer (30-40 microns) co-sintered between the meshsupport layer and the electrolyte membrane layer (see FIGS. 3 and 4).The entire active cell area of the HybridCell has a thin electrolytemembrane (10-20 microns) and the periphery of the cell is dense. Thecathode layers are subsequently deposited onto the major faces withinthe active cell region to complete the fabrication of an SOFC based onthe HybridCell structure. Specific advantages of these two types ofcells are summarized below:

Thin Electrolyte Membrane Layers for High Performance. Both the FlexCelland HybridCell feature a thin electrolyte membrane layer (10-40microns), which minimizes electrolyte ohmic losses at lower operatingtemperatures. Thus, SOFC performance levels achieved with these cellscan be made equivalent to those of anode supported cells.

Small Repeat Units for High Power Density. The total thickness of theFlexCell and HybridCell are less than 200 microns after deposition ofelectrodes, which compares to more than 600-1000 microns thickness ofconventional anode supported cells. This greatly reduces size and weightcontributions of the cells to the total stack weight and volume. Thus,high power density SOFC stacks can be constructed.

Mechanical Strength and Flexibility. The use of partially stabilizedzirconia (yttria or scandia doped) as the mesh support component of thecell results in high mechanical strength, which makes the cell easier tohandle during stack fabrication operations. This also reduces the amountof pressurization required during stack operation because pressure isrequired only to maintain gas-tight seals and not to keep the cells flat(as is the case with anode supported cells).

Dense Perimeter for Ease of Sealing. The dense electrolyte perimeter ofthe FlexCell and HybridCell structures aids sealing. The sealingsurfaces are dense and made of a relatively inert electrolyte materialrather than porous and made of a relatively reactive electrode material.

Thin Anode for Redox and Thermal Cycling Tolerance. The thin anode layerintrinsic to the FlexCell and HybridCell structures makes it much easierto produce cells that are tolerant to both redox and thermal cycling.Thus, excessive measures are not required to maintain the anode in itsfully reduced state during transient operation of SOFC stacks.

Anode Material Flexibility. A specific advantage of the FlexCell designis that the anodes are deposited in a separate manufacturing operation.This makes it easy to incorporate new anode materials that providegreater functionality (e.g., anodes that are tolerant to sulfurimpurities).

For SOFCs to be of practical application, they must operate using fuelsthat are easily available. This requires that power supplies operate onconventional fuels, such as gasoline, natural gas, and diesel. Thehydrocarbon fuel is pre-reacted (reformed) over a catalyst with airand/or steam to produce a mixture of H₂ and CO (and in some cases CH₄)gas before delivery to the fuel cell. Promising development is underwayto provide compact and lightweight reformers for conventional fuels.However, traditional fuels contain some level of sulfur. Sulfur can havedevastating effects on conventional SOFC performance. Cermet mixtures ofnickel metal with electrolyte materials (YSZ or GDC) are the most commonSOFC anodes, but are susceptible to sulfur poisoning in concentrationsas low as a few ppm. This leads to significant performance degradation,especially at lower operating temperatures (700-800° C.) which aredesired for SOFC stacks that use inexpensive metallic interconnectcomponents. Nickel-based cermet anodes experience a two-stagedeactivation when exposed to sulfur (see FIG. 5). The followingmechanisms have been proposed:

Stage I Degradation: The first stage of anode degradation ischaracterized by a rapid drop in cell performance upon introduction ofsulfur to the fuel and is nearly instantaneous. Stage I degradation islargely recoverable upon removal of the sulfur contaminant. The Stage Idegradation process is theorized to be related to the coverage of activenickel sites (at triple-phase boundary regions) with sulfide. Thereactions that occur in Ni/YSZ anodes are shown below:NiO+H₂

Ni+H₂O ΔG at 750° C.=−44 kJ/mol  (1)3Ni+2H₂S

Ni₃S₂+2H₂ ΔG at 750° C.=−68 kJ/mol  (2)While bulk nickel sulfide formation is not favored in low concentrationsof H₂S (<2000 ppm), sulfidation of small nickel particles and roughsurfaces does proceed readily even in very low H₂S concentrations.Surface sulfidation (but not in the bulk) of nickel to Ni₃S₂ has beenobserved experimentally with Raman spectroscopy by analyzing a Ni/YSZcermet exposed to 100 ppm H₂S.

Stage II Degradation: The second stage is characterized by a slowerdegradation of cell performance, which is not recoverable. Someresearchers have reported a cascading effect at longer times. Themechanism of this degradation is theorized to be due to a loss of nickelsurface area through surface rearrangement and sintering of the nickelparticles. Nickel sulfide (Ni₃S₂) has a melting point of 787° C.; thus,sulfide formation could contribute extensively to nickel sintering.

Desulfurizers are being developed to protect fuel cell anodes fromsulfur but they are too large, heavy and complex for many applications,accounting for 10-50% of total system weight, depending on the targetedfuel and useful desulfurizer life. Additionally, desulfurizers add costand maintenance requirements to SOFC systems. An ideal SOFC system wouldtolerate sulfur without the cost, weight, and volume of the desulfurizeralthough for certain applications inclusion of a desulfurizer still maybe preferred. In such systems, the capability of SOFC anodes to resistdegradation by sulfur will offer an opportunity to minimize thedurations between replacement of sulfur adsorbent beds, so that all ofthe desulfurizer capacity can be used and service costs reduced. Sulfurtolerant SOFC anodes therefore are a critical and enabling technologyneed. One embodiment of the present invention is an entirely newapproach to achieving sulfur tolerance in solid oxide fuel cells. Anovel anode materials system, based on commonly used SOFC materials andmethods, provides the unique capability to achieve sulfur tolerance inSOFCs without sacrificing power density, resorting to excessively highSOFC operating temperatures, or adding significant cost.

A key to controlling cost in planar solid oxide fuel cell stacks is theuse of low-cost metals for the interconnect components. In addition tolow cost, there are a number of technical requirements of metallicinterconnect materials, including but not limited to thermal expansionmatch with SOFC materials, high oxidation resistance at elevatedtemperatures in oxidizing environments, and the ability to use low costfabrication methods (e.g., rolling and stamping) to fabricateinterconnect materials of desired shapes. Many alloys have beenevaluated but only a few have been shown to possess the desiredproperties. In particular, chromium-containing ferritic alloys (e.g.,Crofer 22-APU, E-Brite, SS-441, and others known in the art) have shownpromise for SOFC applications. Although these alloys are not completelyimmune to oxidation at SOFC operating temperatures (i.e., on the cathodefaces), the scale that forms during oxidation is itself electricallyconducting. However, these alloys do show a tendency for chromeevaporation during SOFC operation, with chrome migrating to the cathodematerial and causing deterioration of cathode performance. For thisreason, considerable effort has been focused on the development ofcoatings that could be applied to the cathode faces of the metallicinterconnects for the dual purposes of further suppressing alloyoxidation or suppressing migration of the volatilized chrome species.Although progress is being made in development of such coatings, anotherembodiment of this invention is a novel cathode materials system forstabilizing cathode materials against the deleterious effect of chromepoisoning.

BRIEF DESCRIPTION OF THE DRAWINGS

These and further objects of the invention will become apparent from thefollowing detailed description.

FIG. 1 is a picture of a planar electrolyte membrane based on theFlexCell structure.

FIG. 2 is a schematic diagram of the FlexCell architecture.

FIG. 3 is a picture of an anode/electrolyte bi-layer membrane based onthe HybridCell structure.

FIG. 4 is a schematic diagram of the HybridCell architecture.

FIG. 5 is a plot of SOFC data obtained at 750° C. for the solid oxidefuel cell of Example 4 having a cobalt-doped SDC interlayer and aconventional Ni/GDC anode, showing two modes of deactivation caused bythe introduction of H₂S.

FIG. 6 is a comparison of button-cell SOFC data obtained at 800° C. forFlexCell based solid oxide fuel cells of Examples 6 and 7, showing theperformance advantages obtained by replacing the conventional Ni/YSZanode with the high-performance, multilayer anode system of the presentinvention.

FIG. 7 is a schematic diagram of the high-performance andsulfur-tolerant anode design of the present invention.

FIG. 8 shows plots of electrical conductivity of samarium-doped ceria(SDC) in air and reducing atmospheres (top) and SOFC data obtained viabutton cell testing of an electrolyte supported solid oxide fuel cellwith a pure ceramic gadolinium-doped ceria (GDC) anode at 850° C.(bottom).

FIG. 9 is a comparison of area-specific resistance of anodes at 800° C.,measured by electrochemical impedance spectroscopy with various sulfurcontents, showing the importance of the ceria anode/electrolyteinterfacial layer for reducing the level of Stage I deactivation uponintroduction of sulfur: the FlexCell based solid oxide fuel cell ofExample 6 having a multilayer Ni—Co/SDC anode with a cobalt-doped SDCanode/electrolyte interfacial layer (left); and the FlexCell based solidoxide fuel cell of Example 9 having a Ni—Co/SDC anode without acobalt-doped SDC anode/electrolyte interfacial layer (right).

FIG. 10 is a comparison of button-cell SOFC data obtained at 750° C. forelectrolyte-supported solid oxide fuel cells with the same LSM-basedcathode system and different anodes: the solid oxide fuel cell ofExample 4 having a cobalt-doped SDC anode/electrolyte interfacial layerand a Ni/GDC anode; and the solid oxide fuel cell of Example 5 having acobalt-doped SDC anode/electrolyte interfacial layer and a Ni—Co/GDCanode.

FIG. 11 is a comparison of button-cell SOFC data obtained at 800° C. forFlexCell based solid oxide fuel cells having identical LSZF-basedcathodes and different anodes, which shows the effects of varyingamounts of H₂S (0-200 ppm) on voltage stability of cells operating at800° C. with a constant current density of 0.5 A/cm²: the solid oxidefuel cell of Example 7 having a traditional Ni/YSZ anode and not havinga ceria interfacial layer (top); and the solid oxide fuel cell ofExample 6 having a cobalt-doped SDC anode/electrolyte interfacial layer,a Ni—Co/SDC active anode layer, and a Ni—Co/ScSZ current carrying anodelayer (bottom).

FIG. 12 provides long-term, single-cell SOFC testing data obtained on alarge-area FlexCell based solid oxide fuel cell of Example 11, with thehigh-performance, multilayer anode of one embodiment of the invention,showing the effects of 200 ppm H₂S on voltage stability during constantcurrent testing (0.7 A/cm²) at temperatures of 750, 800 and 850° C.

FIG. 13 is a plot of long-term, single-cell testing data obtained onlarge-area HybridCell based solid oxide fuel cell of Example 13, with amultilayer cathode materials system comprising a cobalt-doped SDCcathode/electrolyte interfacial layer, an electrochemically active(LSZF/GDC/Pd) cathode layer, and a current carrying (LSZF/Pd) cathodelayer, demonstrating exceptional voltage stability during constantcurrent testing (0.71 A/cm²) at 750° C.

FIG. 14 is a comparison of x-ray diffraction patterns of calcined(Ni_(0.76)Co_(0.24))O solid solution powder and the reducedNi_(0.76)Co_(0.24) alloy powder of Example 1.

FIG. 15 is a comparison of button-cell SOFC data obtained at 800° C. forthe FlexCell based solid oxide fuel cell of Example 6, showing theeffect of 200 ppm H₂S on power density versus current density curves.

FIG. 16 is a comparison of button-cell SOFC data obtained at 800° C. forFlexCell based solid oxide fuel cells of Examples 8 (top), 9 (middle),and 10 (bottom), showing the effects of 200 ppm H₂S on power densityversus current density curves.

FIG. 17 is a comparison of button-cell SOFC data obtained at 800° C. forFlexCell based solid oxide fuel cells of Examples 8 (top), 9 (middle),and 10 (bottom), showing the effects of varying amounts of H₂S (0-200ppm) on voltage stability of cells operating at 800° C. with a constantcurrent density of 0.5 A/cm².

FIG. 18 is a plot of SOFC performance data (cell potential and powerdensity versus current density) at temperatures of 750, 800 and 850° C.for the large-area FlexCell based solid oxide fuel cell of Example 12,measured under conditions of low fuel use.

FIG. 19 is a plot of SOFC performance data (cell potential versuscurrent density) at temperatures of 750 and 800° C. for the large-areaFlexCell based solid oxide fuel cell of Example 12, measured underconditions of high fuel use.

FIG. 20 is a plot of cell voltage versus time obtained during thermalcycling testing of the larger-area FlexCell-based solid oxide fuel cellof Example 14, which was prepared without nanoscale SDC in theelectrochemically active layer of the anode materials system.

FIG. 21 is a plot of cell voltage versus time obtained during thermalcycling testing of the larger-area FlexCell-based solid oxide fuel cellof Example 15, which was prepared with nanoscale SDC in theelectrochemically active layer of the anode materials system.

FIG. 22 is a comparison of button-cell SOFC data obtained at 600° C.(top) and 650° C. (bottom) for FlexCell-based solid oxide fuel cells ofExamples 16 and 17, both with a gadolinium-doped ceria electrolytematerial, showing the effects of the cobalt-doped SDC interlayer onpower density versus current density curves.

DESCRIPTION OF THE INVENTION

The present invention provides electrode materials systems for planarsolid oxide fuel cells with high electrochemical performance. The novelanode materials provide exceptional long-term durability when used inreducing gases. The novel cathode materials provide exceptionallong-term durability when used in oxygen-containing gases.

The present invention provides novel cermet anode materials in which themetal component of the cermet is a cobalt-nickel alloy rather thannickel metal. These anode materials provide exceptional long-termdurability when used in reducing gases, e.g., in SOFCs with sulfurcontaminated fuels. The present invention also provides a cermet anodehaving a mixed-conducting ceria-based electrolyte material rather than aconventional purely ionically conducting doped zirconia electrolytematerial. In addition, the present invention provides a bi-layer anodestructure to separate and optimize electrochemical activity, gastransport, and current collection functions. The present inventionfurther provides a cerium oxide based interfacial layer with mixedelectronic and ionic conduction at the electrolyte/anode interface.Although each of the foregoing aspects of the invention may be usedindependently or in combinations of less than all of the aspects, thecombination of all of these aspects may offer particular advantages.

The present invention provides novel cathode materials comprisingzinc-doped lanthanum strontium ferrite (LSZF) or an alternative ferrite,cobaltite or nickelate ceramic electrode material rather thanconventional lanthanum strontium manganite (LSM). The cathode materialsalso may comprise a mixed-conducting ceria-based electrolyte materialrather than a purely ionically conducting doped zirconia electrolytematerials used in conventional composite cathodes. The cathode materialsalso may comprise a palladium dopant. In addition, the present inventionprovides a bi-layer cathode structure to separate and optimizeelectrochemical activity, gas transport and current collectionfunctions. The present invention further provides a ceria-basedinterfacial layer with mixed electronic and ionic conduction at theelectrolyte/cathode interface. Although each of the foregoing aspects ofthe invention may be used independently or in combinations of less thanall of the aspects, the combination of all of these aspects results inan SOFC cathode with superior electrochemical performance compared toconventional LSM based cathodes used in oxygen containing gases. Thismultilayer cathode system and its palladium doped cathode material alsoexhibit a high degree of tolerance to chromium contamination duringoperation with metallic interconnect materials.

High Performance, Multilayer Anode System

The high performance, multilayer anode system disclosed hereinincorporates four major embodiments: (1) a cerium oxide basedinterfacial layer with mixed electronic and ionic conduction at theelectrolyte/anode interface; (2) a bi-layer anode structure to separateand optimize electrochemical activity, gas transport and currentcollection functions; (3) a cermet anode in which the electrolytematerial comprises a mixed-conducting ceria-based electrolyte material;and (4) a cermet anode in which the metal component comprises acobalt-nickel alloy. Combined, these four embodiments produce an SOFCanode with superior electrochemical performance compared to conventionalNi/YSZ cermet anodes (see FIG. 6). Further, this multilayer anode systemexhibits a high degree of sulfur tolerance. The structure of themultilayer anode system is depicted in FIG. 7 and described in detailbelow.

More specifically, the invention provides a sulfur tolerant anodematerial for use in an electrochemical device such as a solid oxide fuelcell. The anode material may be a cermet in which the metallic componentcomprises at least one of nickel, an alloy containing nickel and cobalt,and a mixture of nickel and cobalt compositions, and the ceramiccomponent comprises a mixed conducting electrolyte material. The mixedconducting electrolyte material may be a doped ceria electrolytematerial, a doped zirconia electrolyte material, a lanthanum strontiummagnesium gallium oxide (LSGM), another oxygen-ion-conducting ceramicelectrolyte material, or a mixture thereof. The doped ceria electrolytematerial may be a gadolinium doped ceria, a samarium doped ceria, azirconium doped ceria, a scandium doped ceria, a yttrium doped ceria, acalcium doped ceria, a strontium doped ceria, cerium oxide doped with atleast one element selected from rare earth and alkaline earth elements,or a combination thereof. The doped zirconia electrolyte material may bea yttrium doped zirconia, a scandium doped zirconia, a calcium dopedzirconia, zirconium oxide doped with at least one element selected fromrare earth and alkaline earth elements, or a combination thereof. Thecermet may comprise 20 to 80 percent by weight of an ceramic electrolytematerial selected from gadolinium doped ceria and samarium doped ceria.

The invention also provides an anode for a solid oxide fuel cell, theanode comprising a cermet in which the metallic component comprises atleast one of nickel, an alloy containing nickel and cobalt, and amixture of nickel and cobalt compositions and the ceramic componentcomprises a mixed conducting electrolyte material, with nanoscaleparticles of a ceramic material being resident within the grains of themetallic component. The nano scale ceramic material may be a dopedzirconia electrolyte material, a doped ceria electrolyte material, or acombination thereof. The anode may be prepared by the process ofselecting a powdered anode precursor from nickel oxide, a solid solutionof nickel oxide and at least one other metallic oxide, a compoundincluding nickel oxide and at least one other metal oxide, and mixturesthereof, wherein the at least one other metal oxide is selected fromcobalt oxide, iron oxide, copper oxide, and mixtures thereof; combiningthe powdered anode precursor with a nanoscale ceramic material toproduce a powdered anode precursor/nanoscale ceramic materialcombination; calcining the powdered anode precursor/nanoscale ceramicmaterial combination; mixing the calcined powdered anodeprecursor/nanoscale ceramic material combination with a ceramicelectrolyte powder; depositing the mixture on a substrate; sintering thedeposited material; and reducing the sintered material in the presenceof hydrogen or a reducing gas mixture. The process further may comprisethe steps of selecting the nanoscale ceramic material from a dopedzirconia electrolyte material and a doped ceria electrolyte materialand/or selecting the ceramic electrolyte powder from a doped zirconiaelectrolyte material, a doped ceria electrolyte material, andcombinations thereof.

The invention also provides a bi-layer anode/electrolyte interfaciallayer for separating an anode from an electrolyte membrane in a solidoxide fuel cell. The anode/electrolyte interfacial layer may comprise afirst thin dense ceria layer on the surface of a ceramic electrolytemembrane and a second thin porous ceria layer on the dense ceria layer.The thin dense ceria layer may comprise a doped ceria electrolytematerial and the thin porous ceria layer may comprise a doped ceriaelectrolyte material. The ceramic electrolyte membrane may comprise adoped zirconia electrolyte material. At least one of the ceria layersmay be doped with cobalt.

In addition, the invention provides an anode for a solid oxide fuel cellfor use with sulfur-containing fuel streams, the anode comprising afirst cermet anode layer on an electrolyte layer, the metallic componentof the first cermet comprising at least one of nickel, an alloycontaining nickel and cobalt, and a mixture of nickel and cobaltcompositions, and the ceramic component of the first cermet comprising amixed conducting electrolyte material and a second cermet anode layer onthe first cermet anode layer, the metallic component of the secondcermet comprising at least one of nickel, an alloy containing nickel andcobalt, and a mixture of nickel and cobalt compositions, the ceramiccomponent of the second cermet comprising a ceramic electrolytematerial, and the cermet having a coarser microstructure and a highermetal content than the first cermet layer. The mixed conductingelectrolyte material in the first cermet anode layer may be a dopedceria electrolyte material and the ceramic electrolyte material in thesecond cermet layer may be a doped zirconia electrolyte material, adoped ceria electrolyte material, or a combination thereof. The anodemay have nanoscale particles of a ceramic material resident within thegrains of the metallic component of at least one of the cermet layers,the nanoscale ceramic material being selected from a doped ceriaelectrolyte material, a doped zirconia electrolyte material, andcombinations thereof.

The invention also provides an anode system for a solid oxide fuel cellfor use with sulfur-containing fuel streams, the anode comprising afirst anode/electrolyte interfacial layer comprising a thin dense cerialayer on an electrolyte membrane, a second anode/electrolyte interfaciallayer comprising a porous ceria layer on the first anode/electrolyteinterfacial layer. a first cermet anode layer on the secondanode/electrolyte interfacial layer, the metallic component of the firstcermet comprising at least one of nickel, an alloy containing nickel andcobalt, and a mixture of nickel and cobalt compositions, and the ceramiccomponent of the first cermet comprising a mixed conducting electrolytematerial, and a second cermet anode layer on the first cermet anodelayer, the metallic component of the second cermet comprising at leastone of nickel, an alloy containing nickel and cobalt, and a mixture ofnickel and cobalt compositions, the ceramic component of the secondcermet comprising a ceramic electrolyte material, and the cermet havinga coarser microstructure and a higher metal content than the firstcermet layer. At least one of the anode/electrolyte interfacial layersmay comprise a doped ceria electrolyte material. Alternatively, at leastone of the anode/electrolyte interfacial layers may comprise a ceriaelectrolyte material doped with cobalt. Nanoscale particles of a ceramicmaterial may be resident within the grains of the metallic component ofat least one of the cermet layers, the nanoscale ceramic material beingselected from a doped ceria electrolyte material, a doped zirconiaelectrolyte material, and combinations thereof.

The invention also provides a solid oxide fuel cell for use with asulfur containing fuel stream. The solid oxide fuel cell may comprise aceramic electrolyte membrane, a bi-layer anode/electrolyte interfaciallayer on one face of the ceramic electrolyte membrane, an anode appliedto the bi-layer anode/electrolyte interfacial layer, the anodecomprising a cermet in which the metallic component comprises at leastone of nickel, an alloy containing nickel and cobalt, and a mixture ofnickel and cobalt compositions, and the ceramic component is a mixedconducting ceramic electrolyte material, and a cathode applied to theopposite face of the ceramic electrolyte membrane. The bi-layeranode/electrolyte interfacial layer may comprise a first thin denseceria layer on the ceramic electrolyte membrane surface and a thinporous ceria layer on the thin dense ceria layer. The anode may comprisea first anode layer on the porous ceria layer and a second anode layeron the first anode layer, the first anode layer comprising a cermet inwhich the metallic component comprises at least one of nickel, an alloycontaining nickel and cobalt, and a mixture of nickel and cobaltcompositions, and the ceramic component is a ceria-based electrolytematerial, and having a having a fine-scale microstructure and the secondanode layer comprising a cermet in which the metallic componentcomprises at least one of nickel, an alloy containing nickel and cobalt,and a mixture of nickel and cobalt compositions, and a ceramic componentcomprising a ceramic electrolyte material, and having a coarsermicrostructure and a higher nickel content than the first anode layer.Nanoscale particles of a ceramic electrolyte material are residentwithin the metallic component of at least one of the cermet anodelayers, the nanoscale electrolyte material being selected from a dopedzirconia material, a doped ceria material, and combinations thereof.

The invention also provides a solid oxide fuel cell for use with areducing gas, the solid oxide fuel cell comprising a ceramic electrolytemembrane, an anode interfacial layer comprising a thin dense ceria layeron the ceramic electrolyte membrane surface and a porous ceria layer onthe thin dense ceria layer, an anode comprising a first anode layer onthe porous ceria layer and a second anode layer on the first anodelayer, the first anode layer comprising a cermet in which the metalliccomponent comprises at least one of an alloy containing nickel andcobalt and a mixture of nickel and cobalt compositions, the ceramiccomponent comprises a mixed conducting ceria-based electrolyte material,and having a fine-scale microstructure; the second anode layercomprising a cermet in which the metallic component comprises at leastone of nickel, an alloy containing nickel and cobalt, and a mixture ofnickel and cobalt compositions, the ceramic component comprises aceramic electrolyte material, and having a coarser microstructure and ahigher nickel content than the first anode layer; nanoscale particles ofa ceramic electrolyte material being resident within the grains of themetallic component of at least one of the anode layers, and a cathode onthe ceramic electrolyte membrane. Alternatively, the solid oxide fuelcell for use with a reducing gas may comprise a ceramic electrolytemembrane, an electrolyte/anode interfacial layer comprising a thin denseceria layer on the ceramic electrolyte membrane surface and a porousceria layer on the thin dense ceria layer, a bi-layer anode comprising afirst anode layer on the porous electrolyte/anode interfacial layer anda second anode layer on the first anode layer, the first anode layercomprising a cermet in which the metallic component comprises at leastone of nickel, an alloy containing nickel and cobalt, and a mixture ofnickel and cobalt compositions and the ceramic component comprises amixed conducting ceria-based electrolyte material and having afine-scale microstructure; the second anode layer comprising a cermet inwhich the metallic component comprises at least one of nickel, an alloycontaining nickel and cobalt, and a mixture of nickel and cobaltcompositions and the ceramic component comprises a ceramic electrolytematerial and having a coarser microstructure and a higher nickel contentthan the first anode layer, and wherein nanoscale particles of a ceramicelectrolyte material are resident within the grains of the metalliccomponent of at least one of the anode layers, an electrolyte/cathodeinterfacial layer comprising a first thin dense ceria layer on theopposing side of the ceramic electrolyte membrane and a second thinporous ceria layer on the thin dense ceria layer, and a bi-layer cathodecomprising a first cathode layer on the porous electrolyte/cathodeinterfacial layer and a second cathode layer on the first cathode layer,the first cathode layer comprising a composite of a mixed conductingceria-based electrolyte material and a ceramic electrode material andhaving a fine microstructure and small-scale porosity and the secondcathode layer comprising a single phase ceramic electrode material andhaving a coarser microstructure and larger scale porosity than the firstcathode layer.

Anode/Electrolyte Interfacial Layer:

An interfacial layer of a ceria-based electrolyte material with mixedionic and electronic conductivity and a tailored microstructure isdeposited at the anode/electrolyte interface. The interfacial layer isdeposited in two steps, whereby the first step provides a thin denseceria film on the electrolyte membrane surface and the second stepprovides a porous ceria film with an intentionally textured surface onthe thin dense ceria film. The total ceria interlayer thickness isapproximately 2-3 microns. Densification and adhesion of the two ceriainterfacial layer coatings during annealing are enhanced by doping withcobalt. The dense portion of the layer changes the nature of theelectrolyte surface, leaving only mixed conducting interfaces betweenthe electrolyte and anode, while extending the active electrode area.The porous portion of the interfacial layer increases the volume oftriple-phase boundary regions at the anode/electrolyte interface thusreducing resistance to oxygen transport and provides a rough surfacethat provides for good adhesion of subsequently deposited anodecoatings. The high level of oxygen ion conductivity is critical topreventing sulfidation of surface nickel sites in the active anoderegion. Oxidation of nickel sulfide to SO_(x) is a critical reaction tomaintain high anode activity in the presence of sulfur. Table 1 showsthe high dependence of sulfide oxidation on temperature relative toother anode reactions, explaining the well-known relation of improvedsulfur tolerance with increasing temperature.

TABLE 1 Gibbs free energies of reactions involving sulfur species.□G_(RXN) at T (° C.) Reaction 750 800 850 (1) 3Ni + 2H₂S = Ni₃S₂ + 2H₂−68 −65 −62 (2) Ni₃S₂ + O₂ = 3Ni + 2SO 48 22 −4 (3) 2CeO_(1.72) +1.44H₂S = Ce₂O₂S + 1.44H₂O −10 −12 −14 (4) Ce₂O₂S + O₂ = Ce₂O₃ + SO −162−163 −164 (5) SO + ½O₂ = SO₂ −224 −220 −216

Reaction 2, above, is more favorable at higher temperatures and canproceed quickly only if the active nickel phase is in direct contactwith a high-conductivity mixed conductor. Thus, the mixed conductingceria interfacial layer provides for the facile oxidation of sulfidednickel in the active anode region. A typical material with high mixedconductivity in reducing atmospheres is cerium oxide doped withsamarium, gadolinium, or other rare earth and/or alkaline earthelements. The high conductivity of samarium-doped ceria (SDC) inreducing atmospheres is shown in FIG. 8. The electronic conductivity ofSDC in the active anode layer extends electron transfer by increasingtriple-phase boundary area into the anode from the anode/electrolyteinterface. Because of the mixed conductive and catalytic nature ofceria, it also participates in anode reactions. The activity ofgadolinium-doped ceria (GDC) as an anode for hydrogen oxidation is shownby data in FIG. 8. Although the performance of the pure GDC anode doesnot match that of Ni/zirconia or Ni/ceria cermet anodes, its activity issubstantial enough to contribute to anode performance.

Although the use of a ceria interfacial layer provides mixedconductivity at the interface between the anode and electrolyte, whichincreases anode activity, a more important benefit is its positiveimpact on sulfur tolerance. The improvement of anode sulfur toleranceobtained with the SDC interfacial layer is shown by anode resistancemeasurements made using electrochemical impedance spectroscopy (EIS) inFIG. 9. Anode ASR is lower when the ceria (SDC) interfacial layer isincluded and the increase of anode ASR in the presence of 200 ppm H₂S issignificantly smaller than without the ceria interfacial layer.

Bi-Layer Anode Structure:

A bi-layer cermet anode structure optimizes the electrochemical andcurrent carrying performance of the anode system. The metallic stage ofthe cermet may comprise at least one of nickel, an alloy containingnickel and cobalt, a mixture of nickel and cobalt compositions. Theelectrochemically active layer (deposited onto the ceria interfaciallayer) has a fine-scale microstructure and incorporates ceria-basedelectrolyte material as the ceramic phase and a cobalt-nickel alloy asthe metallic phase. The current collecting anode layer has a coarsermicrostructure with higher base metal (nickel and cobalt) content toprovide low resistance to electrical current flow. The composition ofthe ceramic electrolyte material in the current collecting cermet anodelayer need not be the same as the composition of the ceramic electrolytematerial in the active anode layer. In fact, it may be preferable forthe electrolyte material in the current carrying layer to be a zirconiaelectrolyte material, to reduce thermal expansion mismatch with theprimary structural support material in the cell (i.e., the dopedzirconia ceramic electrolyte material). The active and current carryinganode layers are each approximately 15 microns thick.

Mixed Conducting Ceria Electrolyte Phase in Cermet Anode:

The mixed-conducting ceria-based electrolyte in the cermet anode notonly enhances electrochemical performance but also is important tosulfur tolerance. The mixed conductive cermet provides more active sitesfor sulfur oxidation and more facile nickel sulfide oxidation for thesame reasons described in the above discussion of the ceria interfaciallayer. The mixed conducting electrolyte material may be a doped ceriaelectrolyte material, a doped zirconia electrolyte material, a lanthanumstrontium magnesium gallium oxide (LSGM), another oxygen-ion-conductingceramic electrolyte material, and mixtures thereof. The doped ceriaelectrolyte material may be a gadolinium doped ceria (GDC), a samariumdoped ceria (SDC), a zirconium doped ceria, a scandium doped ceria, ayttrium doped ceria, a calcium doped ceria, a strontium doped ceria, acerium oxide doped with at least one element selected from rare earthand alkaline earth elements, and combinations thereof, and the dopedzirconia electrolyte material may be a yttrium doped zirconia (YSZ), ascandium doped zirconia (ScSZ), a calcium doped zirconia, a zirconiumoxide doped with at least one element selected from rare earth andalkaline earth elements, and combinations thereof. For example, a cermetuseful in a sulfur tolerant anode material may comprise 20-80 wt %ceramic electrolyte material selected from a gadolinium doped ceria anda samarium doped ceria. The ceramic electrolyte materials may includeother dopants so long as they do not interfere with electrolytefunction.

Multi-Component Metal Phase:

Binary Co—Ni alloys as the metallic phase of the cermet anode arecritical to achieving high performance and sulfur tolerance. The highermelting temperature of the Ni—Co alloys and the much higher meltingtemperature of cobalt sulfide appear to significantly reducesteady-state degradation during operation in the presence of sulfur.When exposed to sulfur, electrolyte-supported SOFCs with binary alloycermet anodes (Ni—Co/GDC) exhibit significantly reduced Stage II(steady-state) degradation in direct contrast to SOFCs with moreconventional (Ni/GDC) anodes (see FIG. 10). This observation can beexplained by the hypothesis that the sulfided form of the Ni/Co alloyhas a higher melting point than nickel sulfide. The melting points ofsome common nickel and cobalt oxides and sulfides are shown in Table 2.The melting point of Ni₃S₂, the species detected in poisoned nickelanodes, is only 787° C. In contrast, cobalt sulfide has a significantlyhigher melting point. Thus, one would expect that sulfides of Ni—Coalloys have elevated melting points compared to pure nickel and wouldnot sinter as readily. It also is thought that Ni—Co based anodesprovide improved electrochemical performance for hydrogen oxidationreactions as they present lower resistance to the rate limiting chargetransfer step in hydrogen oxidation. By extension, it is reasonable toexpect that cobalt also contributes to the catalytic oxidation of sulfurcontaining species which has been identified as a critical feature forsulfur tolerance.

TABLE 2 Melting points of common Ni and Co compounds. Compound MeltingPoint (° C.) Ni 1455 NiO 1955 Ni₃S₂ 787 NiS 976 Co 1495 CoO 1830 CoS1182

Combining these four aspects, the impact of the two deactivationprocesses can be reduced and a high degree of sulfur tolerance can beachieved. The level of sulfur immunity is shown by SOFC data obtained onidentical FlexCell-based SOFCs with different anodes in FIG. 11. TheSOFC with the high-performance, multilayer anode system exhibited only a2.5% performance drop with additions of up to 200 ppm sulfur over 100hours while the SOFC with a conventional Ni—YSZ anode failed immediatelyupon introduction of only 20 ppm H₂S. The multilayer anode system issubstantially more tolerant to sulfur than the conventional Ni/YSZanode.

The SOFC results presented above were obtained using “button-cell”testing methods involving small cells (˜1 cm² in active area) and lowfuel use (˜2-3 percent). Single-cell SOFC tests were also completed onlarger area cells (28 cm²) to allow anode performance to be assessedunder realistic conditions. Data presented in FIG. 12 show the effect ofoperating temperature in tests conducted at 0.7 A/cm² with fuel useclose to 50 percent. At 750° C., a slight decrease in voltage (Stage Idegradation) was observed upon introduction of 200 ppm H₂S, followed byvery slow rate of steady-state (Stage II) degradation over 242 hours oftesting. The initial voltage loss was fully recovered when H₂S wasremoved from the fuel feed. At 800° C., the cell exhibited a very slight(and recoverable) voltage loss and no steady-state deactivation wasobserved over 195 hours of testing. H₂S had no effect whatsoever on thecell at 850° C. Power densities were 470 mW/cm² at 750° C., 530 mW/cm²at 800° C., and 575 mW/cm² at 850° C. in 200 ppm H₂S.

Stabilization of Anode Performance in SOFCs

As stated above, sintering of nickel metal particles within an SOFCanode in the presence of sulfur is presumed to be a mechanism for StageII deactivation due to the lower melting temperature of nickel sulfidecompared to nickel metal. Even in the absence of sulfur, a keydegradation mechanisms during steady-state and/or transient operation ofsolid oxide fuel cells is sintering of the base metal particles incermet anodes (either pure nickel or nickel alloys containing cobalt,iron and/or copper), with sintering and grain growth of nickel or nickelalloys being particularly problematic during thermal cycling and/orreduction-oxidation (redox) cycling. The present invention solves thisproblem by providing nanoscale ceramic electrolyte particles residentwithin the nickel metal or nickel-alloy grains to reduce coarseningduring SOFC operation. This may be accomplished, for example, bydispersing a small amount of nanoscale ceramic electrolyte material(either zirconia or ceria based electrolyte materials) into NiO powder(or into other mixed base metal oxide powder mixtures) prior to mixingwith additional electrolyte powder during the anode synthesis step. Thisapproach is applicable to most solid oxide fuel cell designs, includingtraditional anode-supported cells, traditional cathode supported cells,traditional electrolyte-supported cells, and planar cells of theFlexCell and HybridCell designs.

To demonstrate this approach, two planar cells of the FlexCell designhaving active areas of 28 cm² were prepared. One FlexCell was preparedwithout nanoscale SDC in the base metal component of theelectrochemically active anode layer and the other FlexCell was preparedwith nanoscale SDC dispersed into the base metal component of theelectrochemically active anode layer. Both FlexCells had identical ceriainterfacial layers on anode and cathode faces, identical currentcarrying anode layers, and identical bi-layer cathodes. As will bedescribed in more detail later in this application, the FlexCell havingan anode with dispersed nanoscale SDC exhibited superior resistance todegradation during thermal cycling tests.

High-Performance, Multilayer Cathode System

Cathode/Electrolyte Interfacial Layer:

An interfacial layer of a ceria-based electrolyte material with mixedionic and electronic conductivity and a tailored microstructure isdeposited at the cathode/electrolyte interface. The interfacial layer isdeposited in two steps, whereby the first step provides a thin denseceria film on the electrolyte membrane surface and the second stepprovides a porous ceria film with an intentionally textured surface onthe thin dense ceria film. The total ceria interlayer thickness isapproximately 2-3 microns. Densification and adhesion of the two ceriainterfacial layer coatings during annealing are enhanced via doping withcobalt. The dense portion of the layer changes the nature of theelectrolyte surface, leaving only mixed conducting interfaces betweenthe electrolyte and cathode while extending the active electrode area.The porous portion of the interfacial layer increases the volume oftriple-phase boundary regions at the cathode/electrolyte interface andreduces resistance to oxygen transport. The ceria interfacial layer alsois important because it prevents deleterious reactions between theelectrolyte material (zirconia or lanthanum gallate based) andperovskite structured manganite, ferrite and cobaltite based cathodematerials during the final annealing step. This allows higher annealingtemperatures to be used, which aids in achieving a thermally stablecathode microstructure that does not change during SOFC operation atelevated temperature.

Replacement of LSM with Higher Performance Cathode Materials:

The electrochemical performance of conventional lanthanum strontiummanganite (LSM) cathode material in SOFCs for oxygen reduction reactionsis fairly limited at temperatures below about 800° C. It would beadvantageous to use higher performance electrode materials, such as alanthanum strontium zinc ferrite, a lanthanum strontium manganite, alanthanum strontium ferrite (LSF), a lanthanum strontium cobaltite, alanthanum strontium cobalt ferrite, a barium strontium cobalt ferrite, alanthanum strontium nickelate, a samarium strontium cobaltite (SSC),other cathode materials known to those skilled in the art, and mixturesthereof, in the active cathode layer. However, these higher performancematerials may be used only if their stability under operating conditions(and specifically in the presence of volatilized chromium) can bemaintained over several thousands of hours. Of particular interest tothe multilayer cathode system of the present invention is zinc-dopedlanthanum strontium ferrite (LSZF), which has been described in U.S.Pat. No. 6,946,213. However, the teachings of this disclosure wouldapply to any ceramic electrode material that can be used as an SOFCcathode. As noted above, the use of the ceria interfacial layers makesthe use of these advanced cathode materials more practical.

Bi-Layer Cathode Structure:

A bi-layer cathode structure optimizes the electrochemical and currentcarrying performance of the cathode system. The electrochemically activelayer (deposited onto ceria interfacial layer) is a composite mixture ofa ceramic electrode material (e.g., LSM, LSF, LSCF, LSZF, etc.) and aceria-based electrolyte material (e.g., SDC, GDC, etc.) and has afine-scale and porous microstructure, while the current collectingcathode layer is a single-phase ceramic electrode with a coarsemicrostructure and larger-scale porosity to minimize resistance tocurrent and gas flow. The active and current carrying cathode layers areeach approximately 15 microns thick. The composition of the ceramicelectrode material used for the current carrying layer need not be thesame as the composition of the electrode material in the active(composite) cathode layer. For example, one might optimize thecomposition of the ceramic electrode material in the composite layer forelectrochemical activity, and the composition of the current carryingceramic electrode material for electrical conductivity.

Inclusion of Ceria Electrolyte Phase in Active (Composite) CathodeLayer:

In addition to enhancing cathode performance, replacement of YSZelectrolyte material with ceria based electrolyte material in compositecathodes also allows higher annealing temperatures to be used inconjunction with the incorporation of more reactive (and moreelectrochemically active) electrode materials (e.g., ferrites,cobaltites and nickelates) into the active (composite) cathode layer.

Incorporation of Palladium as a Dopant in the Cathode Layer:

A novel approach for stabilizing cathode performance against thedeleterious effects of chrome evaporation involves doping of the cathodematerial with a small amount of palladium. The palladium containingcathode layers are deposited using convention methods such as screenprinting, spraying, or painting, followed by annealing. To demonstratethis approach, a planar cell of the HybridCell structure was preparedwith an active area of 28 cm² and a multilayer cathode was applied,including a ceria interlayer (as described above), an active cathodelayer that was a composite of(La_(0.60)Sr_(0.40))(Fe_(0.90)Zn_(0.10))O_(3-X) (LSZF) andgadolinium-doped ceria (GDC), and a current carrying cathode layer ofpure LSZF. A palladium addition of one weight percent was made to theactive cathode layer and a two weight percent addition of palladium wasmade to the current collecting cathode layer. This HybridCell was testedusing manifolds that were made of a nickel-chrome alloy (Inconel-601).The cell was operated at 750° C. for about a year (8,500) with ahumidified hydrogen fuel gas (350 cc/min of H₂) being fed to the anodeside of the cell and air (1500 cc/min) being fed to the cathode side. Asis shown in FIG. 13, with a constant current of 0.71 A/cm² being appliedto the cell, there was zero degradation of the voltage after an initial“conditioning” period of about 150 hours (which is attributed tostabilization of the anode microstructure). Without the palladiumaddition, one might expect degradation of cell performance due topoisoning of the cathode by chromium.

Palladium may be used as a stabilizer against chromium poisoning in SOFCcathodes other than the LSZF doped cathodes used for the above-describeddemonstration. These cathode compositions may include a lanthanumstrontium manganite, a lanthanum strontium ferrite (LSF), a lanthanumstrontium cobaltite, a lanthanum strontium cobalt ferrite, a bariumstrontium cobalt ferrite, a lanthanum strontium nickelate, a samariumstrontium cobaltite (SSC), other cathode materials known to thoseskilled in the art, mixtures thereof, and a mixture of any of them withLSZF. The palladium stabilizer may be incorporated into the cathodelayers, for example, by mixing palladium metal or palladium oxide withthe cathode powders before preparation of the screen printing ink,incorporating palladium into the cathode material by traditionalincipient wetness methods prior to making the screen printing ink,dissolving palladium salts into the screen printing ink vehicle beforemixing the cathode powder into the ink vehicle, or infiltrating asolution containing a palladium salt into a pre-formed cathode coating.

The anode materials system of the present invention is described in moredetail below. The anode materials system includes a thin interfaciallayer of a mixed ionically and electronically conducting ceramicmaterial that is fabricated in two layers. The mixed ionically andelectronically conducting ceramic material preferably is selected from aceria-based electrolyte material, including a samarium-doped ceria, agadolinium-doped ceria, a yttrium-doped ceria, a scandium-doped ceria, acalcium-doped ceria, and cerium oxide doped with any combination of rareearth and/or alkaline earth elements. These materials may containdopants in addition to those specified so long as these dopants do notinterfere with anode function. Other mixed conducting ceramicelectrolyte materials known to those skilled in the art also may beused. For the case of ceria-based interfacial layers, a first denseceria layer is deposited onto the ceramic electrolyte membrane surfacefrom a mixture of doped cerium oxide electrolyte powders having one ormore primary particle size ranges and a small amount of a sintering aidsuch as cobalt (or alternatively other base metal oxides or a mixture ofbase metal oxides). The second porous layer is deposited from a dopedcerium oxide electrolyte powder having one or more distinct primaryparticle size ranges, a small amount of a sintering aid such as cobalt(or alternatively other base metal oxides or a mixture of base metaloxides), and a fugitive pore forming material (e.g., graphite or othersuitable fugitive materials known to those skilled in the art). The twolayers are deposited sequentially and then co-sintered such that thedoped cerium oxide interfacial layer is dense at the surface of theelectrolyte membrane and porous at the outer surface and the totalthickness of the interfacial layer is approximately 2-3 microns. Both ofthe component layers are deposited from an ink or suspension usingmethods such as screen printing, spraying, painting, or other methodsknown to those skilled in the art. The sintering aid (cobalt or otherbase metal oxide or combination of base metal oxides) may beincorporated during synthesis of the ceria-based powders used to makethe interfacial layer, oxide or metal powders may be pre-mixed withceria-based powders before inks or spray suspensions are made, or thesintering aid may be incorporated during the ink/suspension synthesisstep by pre-dissolving soluble precursors to the sintering aid in theink vehicle (or solvent) that is used to apply the coatings by sprayingor screen printing methods. Other approaches to incorporate sinteringaids to the ceria interfacial layer also may be used.

The anode materials system of the present invention also includes abi-layer anode structure such that the electrochemical and currentcarrying functions of the anode system are separately optimized. Theelectrochemically active anode layer is first deposited onto the ceriainterfacial layer from mixtures of a ceria-based electrolyte powder anda suitable base metal oxide powder (e.g., nickel oxide) or a combinationof pre-calcined base metal oxide powders (e.g., nickel, cobalt, copperand/or iron oxides). The ceria-based electrolyte material in the mixtureused to make the electrochemically active layer may be selected from asamarium-doped ceria, a gadolinium-doped ceria, a yttrium-doped ceria, ascandium-doped ceria, a calcium-doped ceria, and cerium oxide doped withany combination of rare earth and/or alkaline earth elements. Thesematerials may contain dopants in addition to those specified so long asthese dopants do not interfere with anode function. The base metal oxidepowder in the mixtures used to make the electrochemically active anodelayer is comprised of one or more distinct particle size ranges. Theceria-based ceramic electrolyte powder in the mixtures used to make theelectrochemically active anode layer also is comprised of one or moredistinct particle size ranges. The weight ratio of base metal oxidepowder to doped cerium oxide powder in the mixtures used to make theelectrochemically active anode layer can range from 1:1 to 3:1. Thecurrent collecting anode layer is deposited onto the electrochemicallyactive anode layer from mixtures of a ceramic electrolyte powder (e.g.,doped zirconium oxide or doped cerium oxide) and a base metal oxide(e.g., nickel oxide) or combination of base metal oxides (e.g., nickel,cobalt, copper and/or iron oxides). The base metal oxide powders used inthe current carrying anode layer are comprised of one or more distinctprimary particle size ranges, and the weight ratio of base metal oxidepowder to doped cerium oxide powder can range from 3:1 to 5:1. The twotypes of anode layers are deposited and then sintered such that thetotal thickness is approximately 30 microns and each of the componentanode layers is approximately 15 microns thick. In one embodiment of theinvention in which a mixture of base metal oxides (e.g., nickel, cobalt,iron and/or copper) is used, the oxides may be pre-calcined to form asolid solution, a single phase mixed oxide, or a multiple-phase mixedoxide prior to modifying particle size and mixing with the electrolytematerial (ceria or zirconia). The ceria-based anode/electrolyteinterfacial layer, the first (electrochemically active) anode layer andthe second (current carrying) anode layer may be deposited and sinteredseparately or they may be deposited sequentially and then co-sintered.

A small amount of a high surface area, nanoscale electrolyte material(either zirconia or ceria based electrolyte materials) may be mixed intoeither nickel oxide or a mixture of base metal oxides (nickel, cobalt,iron and/or copper oxides) and then calcined before the calcined mixtureis mixed with additional electrolyte material to form a cermet anode.The nanoscale electrolyte material addition prevents sintering of basemetal particles during SOFC operation, during operation with sulfurcontaining fuels, during start-up and shut-down, and duringreduction-oxidation cycling. If doped cerium oxide is used as thenanoscale electrolyte material, then the composition of the nanoscaledoped cerium oxide material may be selected from a samarium-doped ceria,a gadolinium-doped ceria, a yttrium-doped ceria, a scandium-doped ceria,a calcium-doped ceria, and a cerium oxide doped with any combination ofrare earth and/or alkaline earth elements. These materials may containdopants in addition to those specified so long as these dopants do notinterfere with anode function. If doped zirconium oxide is used as thenanoscale electrolyte material, then the composition of the nanoscaledoped zirconia electrolyte material may be a yttrium-doped zirconia, ascandia doped zirconia, and any singly or multiply doped zirconiaelectrolyte material.

The cathode materials system of the present invention includes a thininterfacial layer of a mixed ionically and electronically conductingceramic material which is fabricated in two layers. The mixed ionicallyand electronically conducting ceramic material preferably is selectedfrom a ceria-based ceramic electrolyte material, including asamarium-doped ceria, a gadolinium-doped ceria, a yttrium-doped ceria, ascandium-doped ceria, a calcium-doped ceria, and a cerium oxide dopedwith any combination of rare earth and/or alkaline earth elements. Thesematerials may contain dopants in addition to those specified so long asthese dopants do not interfere with cathode function. Other mixedconducting ceramic electrolyte materials known to those skilled in theart also may be used. For the case of ceria-based interfacial layers, afirst dense ceria layer is deposited onto the ceramic electrolytemembrane surface from a mixture of doped cerium oxide electrolytepowders having one or more primary particle size ranges and a smallamount of a sintering aid such as cobalt (or alternatively other basemetal oxides or a mixture of base metal oxides). The second porous layeris deposited from a doped cerium oxide electrolyte powder having one ormore distinct primary particle size ranges, a small amount of asintering aid such as cobalt (or alternatively other base metal oxidesor a mixture of base metal oxides), and a fugitive pore forming material(e.g., graphite or other suitable fugitive materials known to thoseskilled in the art). The two component layers of the bi-layerinterfacial layer are deposited sequentially and then co-sintered suchthat doped cerium oxide interfacial layer is dense at the surface of theelectrolyte membrane and porous at the outer surface and the totalthickness of the interfacial layer is approximately 2-3 microns. Both ofthe component layers are deposited from an ink or suspension usingmethods such as screen printing, spraying, painting, or other methodsknown to those skilled in the art. The sintering aid (cobalt or otherbase metal oxide or combination of base metal oxides) may beincorporated during synthesis of the ceria-based powders used to makethe interfacial layer by pre-mixing the oxide or metal powdersceria-based powders before inks or spray suspensions are made orincorporating the sintering during the ink/suspension synthesis step bypre-dissolving soluble precursors to the sintering aid oxides in the inkvehicle (or solvent) used to apply the interfacial layer by spraying orscreen printing methods. Other approaches to incorporate the sinteringaid into the ceria-based interfacial layer also may be used.

The cathode materials system of the present invention also includes abi-layer cathode structure such that the electrochemical and currentcarrying functions of the anode system are separately optimized. Theelectrochemically active cathode layer is first deposited onto theceria-based interfacial layer from powder mixtures of a ceria-basedceramic electrolyte material and a ceramic electrode material. Theceramic electrode material may be a lanthanum strontium zinc ferrite(LSZF), a lanthanum strontium manganite, a lanthanum strontium ferrite(LSF), a lanthanum strontium cobaltite, a lanthanum strontium cobaltferrite, a barium strontium cobalt ferrite, a lanthanum strontiumnickelate, a samarium strontium cobaltite (SSC), other cathode materialsknown to those skilled in the art, and a mixture thereof; thesematerials may contain dopants in addition to those specified so long asthese dopants do not interfere with cathode function. The ceramicelectrode powder in the mixtures used to make the electrochemicallyactive cathode layer is comprised of one or more distinct particle sizeranges. The ceria-based electrolyte powder in the mixtures used to makethe electrochemically active cathode layer also is comprised of one ormore distinct particle size ranges. The weight ratio of ceramicelectrode powder to doped cerium oxide powder in the mixtures used tomake the electrochemically active cathode layer can range from 1:2 to2:1. The current collecting cathode layer is either a single-componentceramic electrode material or a ceramic electrode material that is mixedwith a small amount of electrolyte material; this layer is depositedonto the electrochemically active cathode layer. The composition of theceramic electrode material used in the current carrying cathode layermay the same as that used in the electrochemically active cathode layer,or optionally be a different ceramic electrode material selected basedon reasons of higher electrical conductivity, better thermal stability,or improved thermal expansion match with the structural component of thecell (e.g., a zirconia based electrolyte for the case of the FlexCelland HybridCell structures). The ceramic electrode material used in thecurrent carrying cathode layer is comprised of one or more distinctparticle size ranges. The two types of cathode layers are deposited andthen sintered such that the total thickness is approximately 30 micronsand each of the component anode layers is approximately 15 micronsthick. In one embodiment of the invention, a palladium dopant (or otherbase metal or precious metal dopant, or any combination of base metaland precious metal dopants) may be incorporated into at least one of thecathode layers. This dopant addition may be made by re-dissolvingprecursors to the dopant (e.g., acetylacetonates or other solublecompounds) into the ink vehicle (or solvent) that is used to apply thecoatings. The ceria-based cathode/electrolyte interfacial layer, thefirst (electrochemically active) cathode layer and the second (currentcarrying) cathode layer may be deposited and sintered separately or theymay be deposited sequentially and then co-sintered.

The usefulness of the electrode materials systems described in thisinvention are evident from the testing results obtained on solid oxidefuel cells prepared as described in the following Examples.

Example 1

Nickel cobalt oxide solid solution powder was prepared for subsequentuse in anode formulations described in Examples 5, 6, 11, 12, 14, 16,and 17. The first step was the preparation of a mixture that contained112.5 grams nickel oxide (NiO) and 37.50 grams cobalt oxide (Co₃O₄),corresponding to a batched stoichiometry of (Ni_(0.76)Co_(0.24))O. Thismixture was ball milled in acetone with zirconia grinding media and theresulting slurry was dried to a powder. The dried powder mixture wascalcined at 1000° C. and then sieved through a 35-mesh sieve. Duringcalcination, the nickel and cobalt oxides reacted to form a(Ni_(0.76)Co_(0.24))O solid solution powder, as confirmed by x-raydiffraction data in FIG. 14. To determine the effect of reduction on thecrystal structure, a sample of the calcined (Ni_(0.76)Co_(0.24))O powderwas reduced in hydrogen at 800° C. and analyzed by x-ray diffraction(also shown in FIG. 14). These XRD data confirm that the reduced powderis an alloy of nickel and cobalt metals rather than a mixture of nickeland cobalt metals.

The calcined NiO—CoO powder prepared above constituted the “coarse”fraction of NiO—CoO precursor powder. Fine NiO—CoO precursor powder wasmade by the same initial procedure but after calcination the NiO—CoOpowder was vibratory milled in acetone with zirconia grinding media toreduce its particle size. The vibratory milled NiO—CoO slurry then wasdried to complete the preparation of “fine” NiO—CoO precursor powder.

Example 2

NiO—CoO solid solution powder (Ni_(0.76)Co_(0.24)O) containing a smallamount of nanoscale samarium-doped ceria (SDC-15,Ce_(0.85)Sm_(0.15)O_(1.925)) was prepared for subsequent use in anodeformulations described in Examples 6, 8, 10, 11, and 15. The first stepwas the preparation of a mixture that contained 112.5 grams nickel oxide(NiO), 37.50 grams cobalt oxide (Co₃O₄), and 3 grams nanoscale SDC-15powder having a surface area of 195 m²/gram. This mixture was ballmilled in acetone with zirconia grinding media and the resulting slurrywas dried to a powder. The dried NiO—CoO/SDC powder was calcined at1000° C. then sieved through a 35-mesh sieve to complete preparation ofthe coarse NiO—CoO/SDC precursor powder. Fine NiO—CoO/SDC precursorpowder was made by the same initial procedure but after calcination theNiO—CoO/SDC powder was vibratory milled in acetone with zirconiagrinding media to reduce its particle size. The vibratory milledNiO—CoO/SDC slurry then was dried to complete preparation of the fineNiO—CoO/SDC precursor powder.

Example 3

NiO—CoO solid solution powder (Ni_(0.76)Co_(0.24)O) containing smallamounts of nanoscale samarium-doped ceria (SDC-15) and nanoscalezirconium-doped ceria (ZDC-50, Ce_(0.5)Zr_(0.5)O₂) powders was preparedfor subsequent use in an anode formulation described in Example 9. Thefirst step was the preparation of a mixture that contained 112.5 gramsnickel oxide (NiO), 37.50 grams cobalt oxide (Co₃O₄), 1.5 gramsnanoscale SDC-15 powder with a surface area of 195 m²/gram, and 1.5grams nanoscale ZDC-50 powder with a surface area of 81 m²/gram. Thismixture was ball milled in acetone with zirconia grinding media and theresulting slurry was dried to a powder. The dried powder mixture thenwas calcined at 1000° C. then sieved through a 35-mesh sieve. Thecalcined NiO—CoO/SDC/ZDC powder constituted the coarse NiO—CoO/SDC/ZDCprecursor powder. Fine NiO—CoO/SDC/ZDC precursor powder was made by thesame initial procedure but after calcination the NiO—CoO/SDC powder wasvibratory milled in acetone with zirconia grinding media to reduce itsparticle size. The vibratory milled NiO—CoO/SDC/ZDC slurry then wasdried to complete preparation of the fine NiO—CoO/SDC/ZDC precursorpowder.

Example 4

A solid oxide fuel cell was prepared from a 1.9-cm diameterscandia-stabilized zirconia ceramic electrolyte substrate with athickness of approximately 125 microns and an active area of 1.26 cm².The cell was fabricated by screen printing a single-layer, cobalt-dopedsamarium-doped ceria (SDC-20, 20 mole percent samarium) interfaciallayer on both the anode and cathode faces. The cobalt addition was madeby adding cobalt (III) 2,4 pentanedionate to the ink in an amountcorresponding to approximately 1 wt % cobalt metal relative to theamount of SDC-20 powder in the ink. The cobalt-doped SDC interfaciallayers were annealed at 1300° C. for one hour. Thicknesses of theinterfacial layers on both anode and cathode faces were approximatelyfive microns. A bi-layer anode was deposited onto the cobalt doped SDCinterfacial layer on the anode face of the cell. The electrochemicallyactive anode layer comprised a mixture of NiO and gadolinium-doped ceria(GDC-10, 10 mole percent Gd), with 50 wt % GDC-10 and 50 wt % NiO. Thecurrent carrying anode layer comprised a mixture of NiO and GDC-10, with20 wt % GDC-10 and 80 wt % NiO. The two anode layers were annealed at1300° C. for one hour. Thicknesses of the electrochemically active andcurrent carrying anode layers after annealing both were approximately 15microns each. A bi-layer cathode, based on zinc-doped lanthanumstrontium ferrite, (La_(0.60)Sr_(0.40))(Zn_(0.10)Fe_(0.90))O_(3-X)(LSZF) was deposited onto the cobalt-doped SDC interfacial layer on thecathode face of the cell. The electrochemically active cathode layercomprised a mixture of LSZF and GDC-10 (50 wt % LSZF and 50 wt %GDC-10). The current carrying cathode layer was pure LSZF. The twocathode layers were annealed at 1125° C. for one hour. Thicknesses ofthe electrochemically active and current carrying cathode layers afterannealing both were approximately 15 microns each.

The SOFC performance of this cell in the presence of 200 ppm H₂S at 750°C. is shown in FIGS. 5 and 10. Because the anode in this cell did notcontain cobalt, it experienced significant steady-state (Stage II)degradation in the presence of sulfur.

Example 5

A solid oxide fuel cell was prepared from a 1.9-cm diameterscandia-stabilized zirconia ceramic electrolyte substrate with athickness of approximately 125 microns and an active area of 1.26 cm².The cell was fabricated with the same cobalt-doped samarium-doped ceria(SDC) interfacial layers on both the anode and cathode faces asdescribed in Example 4. A bi-layer anode was deposited onto the cobaltdoped SDC interfacial layer on the anode face of the cell. Theelectrochemically active anode layer comprised a mixture of 40 wt %gadolinium-doped ceria (GDC) and 60 wt % coarse NiO—CoO powder preparedas described in Example 1. The current carrying anode layer comprised amixture of NiO and GDC (80 wt % NiO and 20 wt % GDC). The two anodelayers were annealed at 1300° C. for one hour. Thicknesses of theelectrochemically active and current carrying anode layers afterannealing both were approximately 15 microns each. The same LSZF-basedbi-layer cathode as described in Example 4 was deposited and annealedonto the cobalt-doped SDC interfacial layer on the cathode face of thecell.

The SOFC performance of this cell in the presence of 200 ppm H₂S isshown in FIG. 10. Because the anode in this cell contained cobalt, itexperienced lower steady-state degradation in the presence of sulfur,compared to the identical cell without cobalt in the electrochemicallyactive anode layer (Example 4).

Example 6

A solid oxide fuel cell was prepared from a 1.9 cm diameter disc-shapedFlexCell substrate made from scandia-stabilized zirconia (ScSZ-6, sixmole percent Sc₂O₃) electrolyte material with an active area of 0.89cm². The cell was fabricated with cobalt-doped SDC interfacial layers onboth the anode and cathode faces, with a bi-layer anode on the anodeface, and with a bi-layer cathode on the cathode face. This cell wasfabricated as described below:

Deposition of Interfacial Layer Coatings:

Cobalt-doped SDC interfacial layer coatings were prepared according tothe spray deposition methods taught in U.S. patent application Ser. No.11/349,773 (published Aug. 9, 2007). Cobalt-doped SDC interfacial layerink was prepared using samarium-doped ceria powders of the composition(Ce_(0.80)Sm_(0.20))O_(1.90) (SDC-20). The ink was prepared bydispersing SDC-20 powders into a terpineol based an ink vehicle. TheSDC-20 powders in this ink had four different surface areas: 30 percentwith a surface area of 6.0 m²/gram, 40 percent with a surface area of9.3 m²/gram, 20 percent with a surface area of 27 m²/gram, and 10percent with a surface area of 45 m²/gram. A cobalt addition was made byadding cobalt (III) 2,4 pentanedionate in an amount corresponding toapproximately one percent of cobalt metal relative to the total amountof SDC-20 powder in the ink. The cobalt-doped SDC interfacial layer inkwas then made into two separate spray solutions. The first solution wasprepared by diluting a portion of the SDC/Co ink with acetone and thesecond solution was prepared by diluting a portion of SDC/Co ink withacetone and adding 2.5 wt % graphite (solids basis). These solutionswere sprayed onto both sides of the FlexCell substrate. A firstcobalt-doped coating (without graphite) was spray deposited onto oneface of the FlexCell substrate and dried and then the SDC/Co/C coating(with graphite) was spray deposited onto the first coating in the samemanner and dried. The procedure was repeated to deposit a two-layerceria interfacial layer onto the opposite face of the FlexCellsubstrate. The interfacial layer coated FlexCell substrate then washeated in a furnace to 1300° C. for one hour to sinter the interfaciallayer coatings and adhere them to the FlexCell substrate. Spraydeposition parameters were controlled such that the total interfaciallayer thickness was approximately 2-3 microns and each component layerof the interfacial layer was approximately 1-2 microns thick.

Preparation of Electrochemically Active Anode Ink.

NiO—CoO/SDC anode precursor powder prepared as described in Example 2was used to prepare electrochemically active anode inks as follows.Samarium-doped ceria powder of the composition(Ce_(0.9)Sm_(0.10))O_(1.95) (SDC-10) was prepared with different surfaceareas. A mixture was prepared that contained 30 grams coarse NiO—CoO/SDCprecursor powder, 30 grams fine NiO—CoO/SDC precursor, 35 grams SDC-10powder with a surface area of 6.0 m²/gram, and 5 grams SDC-10 powderwith a surface area of 45 m²/gram. This powder mixture then was ballmilled in acetone with zirconia grinding media and dried. A portion ofthis powder was dispersed into a terpineol based ink vehicle to preparethe electrochemically active anode ink.

Preparation of Current Carrying Anode Ink.

NiO—CoO anode precursor powder prepared as described in Example 1 wasused to prepare current carrying anode inks as follows. A mixture wasthen prepared that contained 50 grams coarse NiO—CoO precursor powder,50 grams fine NiO—CoO precursor, and 25 grams scandia-stabilizedzirconia powder (ScSZ, 10 mole percent Sc₂O₃, 3-5 micron particle size).This powder mixture then was ball milled in acetone with zirconiagrinding media and the resulting slurry was dried to a powder. A portionof this powder was dispersed into a terpineol based ink vehicle toprepare the NiO—CoO/ScSZ current carrying anode ink.

Deposition of Anode Coatings.

The two-layer anode was applied to the anode face of theinterfacial-layer-coated FlexCell prepared above. The first,electrochemically active (NiO—CoO/SDC), anode layer was applied bypainting onto the sintered interfacial layer using a foam brush followedby drying. The second, current carrying (NiO—CoO/ScSZ), anode layer wasapplied by painting onto the dried electrochemically active anodecoating using a foam brush followed by drying. The anode-coated FlexCellsubstrate then was heated in a furnace to 1300° C. to sinter the anodelayers and adhere them to the ceria interfacial layer. The amounts ofdeposited anode coatings were controlled such that the total anodethickness was approximately 30 microns and each component layer of thebi-layer anode was approximately 15 microns thick.

Preparation of Electrochemically Active Cathode Ink.

Gadolinium-doped ceria powder of the composition(Ce_(0.9)Gd_(0.10))O_(1.95) (GDC-10) was prepared with different surfaceareas. Zinc-doped lanthanum strontium ferrite of the composition(La_(0.60)Sr_(0.40))(Zn_(0.10)Fe_(0.90))O_(3-X) (LSZF) powder wasprepared with different surface areas. A mixture was prepared thatcontained 125 grams LSZF powder with a surface area of 1.6 m²/gram, 125grams LSZF powder with a surface area of 4.2 m²/gram, 50 grams GDC-10powder with a surface area of 2.9 m²/gram, and 200 grams GDC-10 powderwith a surface area of 8.3 m²/gram. This powder mixture then was ballmilled in acetone with zirconia grinding media and the resulting slurrywas dried to a powder. A portion of this powder was dispersed into anink vehicle along with an amount of palladium 2/4 pentanedionatesufficient to make a 0.35 wt % palladium (relative to total solids inthe ink) to complete preparation of the electrochemically active cathodeink.

Preparation of Current Carrying Cathode Ink.

Zinc-doped lanthanum strontium ferrite of the composition(La_(0.60)Sr_(0.40))(Zn_(0.10)Fe_(0.90))O_(3-X) (LSZF) powder wasprepared with two different surface areas. A mixture was prepared thatcontained 375 grams LSZF powder with a surface area of 2.2 m²/gram and125 grams LSZF powder with a surface area of 4.8 m²/gram. This powdermixture then was ball milled in acetone with zirconia grinding media andthe resulting slurry was dried to a powder. A portion of this powder wasdispersed into an ink vehicle, along with an amount of palladium 2/4pentanedionate sufficient to make a 0.70 wt % palladium (relative tototal solids in the ink), to complete preparation of the currentcarrying cathode ink.

Deposition of Cathode Coatings.

The two-layer cathode was applied to the cathode face of theinterfacial-layer-coated FlexCell prepared above (after anode depositionand sintering). The first, electrochemically active (LSZF/GDC/Pd),cathode layer was applied by painting onto the sintered interfaciallayer using a foam brush and then dried. The second, current carrying(LSZF/Pd), cathode layer was applied by painting onto the driedelectrochemically active cathode coating using a foam brush and thendried. The cathode-coated FlexCell substrate then was heated in afurnace to 1125° C. to sinter the cathode layers and adhere them to theceria interfacial layer. The amounts of deposited cathode coatings werecontrolled such that the total cathode thickness was approximately 30microns and each component layer of the bi-layer cathode wasapproximately 15 microns thick.

The SOFC performance of this cell with hydrogen and air as fuel andoxidant (without H₂S in the fuel) was measured using button-cell testingmethods. The effect of 200 ppm H₂S on the SOFC performance (currentdensity versus power density curves) at 800° C. for this cell is shownin FIG. 15, which further suggests a relatively low degradation in SOFCperformance when H₂S is present in the fuel. Based these and previouslypresented data (shown in FIGS. 6, 9 and 11), it can be concluded thatthis cell, which contains anode and cathode materials of the presentinvention, exhibits the remarkable combination of high SOFC performanceand resistance to degradation by sulfur.

Example 7

A solid oxide fuel cell was prepared from a 1.9 cm diameter disc-shapedFlexCell substrate made from ScSZ-6 electrolyte material with an activearea of 0.89 cm². A conventional Ni/YSZ anode was applied to the anodeface without an interfacial layer and a cobalt-doped ceria interfaciallayer and a bi-layer cathode were applied to the cathode face. This cellwas fabricated as described below:

Deposition of Cathode/Electrolyte Interfacial Layer:

Cobalt-doped SDC interfacial layer inks were prepared and a bi-layerinterfacial layer was applied to the cathode face of a FlexCellsubstrate using the same materials and methods as described in Example6.

Preparation of NiO/YSZ Anode Ink.

Fine NiO anode precursor powder was made by vibratory milling of nickeloxide powder (NiO, Novamet Standard Type) in acetone with zirconiagrinding media, followed by drying. A mixture was prepared thatcontained 50 grams non-milled nickel oxide powder (NiO, Novamet StandardType), 50 grams fine NiO precursor, and 25 grams yttria-stabilizedzirconia powder (YSZ, 8 mole percent Y₂O₃, 3-5 micron particle size).This mixture was ball milled in acetone with zirconia grinding media andthe resulting slurry was dried to a powder. A portion of this powder wasdispersed into a terpineol based ink vehicle to prepare the NiO/YSZanode ink.

Deposition of Ni/YSZ Anode Coating.

The NiO/YSZ anode was applied to the non-interfacial-layer coated anodeface of the FlexCell prepared above. This coating was applied bypainting directly onto the substrate using a foam brush, followed bydrying. The anode-coated FlexCell substrate then was heated in a furnaceto 1300° C. for one hour to sinter and adhere the cobalt-doped ceriainterfacial layer that was previously applied to the cathode face of theFlexCell substrate and to sinter and adhere the anode layer. The amountof anode coating applied was controlled such that the total anodethickness was approximately 20 microns.

Deposition of Bi-Layer Cathode Coatings.

Electrochemically active (LSZF/GDC/Pd) and current carrying (LSZF/Pd)cathode inks were prepared and cathode coatings were deposited onto theceria interfacial layer on the cathode face of the FlexCell using thesame materials, methods and thermal treatments as described in Example6.

The SOFC performance of this cell with hydrogen and air as fuel andoxidant (without H₂S in the fuel) was measured using button-cell testingmethods and is shown in FIG. 6. The adverse effect of 20 ppm H₂S on cellvoltage during constant current (0.5 A/cm²) SOFC testing is shown inFIG. 11. Based on these data, it can be concluded that this cell,without a ceria interfacial layer and with a conventional NiO/YSZ anode,exhibits relatively low SOFC performance and degrades rapidly whensulfur is present in the fuel.

Example 8

A solid oxide fuel cell was prepared from a 1.9 cm diameter disc-shapedFlexCell substrate made from ScSZ-6 electrolyte material with an activearea of 0.89 cm². This cell was the same as Example 6, except that therewas no cobalt in the current carrying layer of the bi-layer anode. Thiscell was fabricated as described below:

Deposition of Interfacial Layer Coatings:

Cobalt-doped SDC interfacial layer inks were prepared and interfaciallayers were applied to both the anode and cathode faces of a FlexCellsubstrate using the same materials, methods and thermal treatments asdescribed in Example 6.

Preparation of Current Carrying Anode Ink.

Fine NiO anode precursor powder was made by vibratory milling of nickeloxide (NiO, Novamet Standard Type) in acetone with zirconia grindingmedia followed by drying. A mixture was prepared that contained 50 gramsnon-milled nickel oxide powder (NiO, Novamet Standard Type), 50 gramsfine NiO precursor, and 25 grams scandia-stabilized zirconia powder(ScSZ, 10 mole percent Sc₂O₃, 3-5 micron particle size. This mixture wasball milled in acetone with zirconia grinding media and the resultingslurry was dried to a powder. A portion of this powder was dispersedinto a terpineol based ink vehicle to prepare NiO/ScSZ current carryinganode ink.

Deposition of Anode Coatings.

A two-layer anode was applied to the anode face of the cobalt-dopedceria interfacial layer on the anode face of the FlexCell substrateusing the electrochemically active (NiO—CoO/SDC) anode ink that wasprepared as described in Example 6 and the current carrying (NiO/ScSZ)anode ink prepared above. The anode coatings were deposited andthermally treated as described in Example 6.

Deposition of Bi-Layer Cathode Coatings.

Electrochemically active (LSZF/GDC/Pd) and current carrying (LSZF/Pd)cathode inks were prepared and cathode coatings were deposited onto thecobalt-doped ceria interfacial layer on the cathode face of the FlexCellsubstrate using the same materials, methods and thermal treatments asdescribed in Example 6.

The SOFC performance of this cell with hydrogen and air as fuel andoxidant (without H₂S in the fuel) was measured using button-cell testingmethods. The effect of 200 ppm H₂S on the SOFC performance (currentdensity versus power density curves) at 800° C. for this cell is shownin FIG. 16. These data confirm that very high performance is obtainedwithout H₂S in the fuel and that a relatively low degradation in SOFCperformance is observed when H₂S is present in the fuel. The effects ofvarying levels of H₂S (20-200 ppm) on cell voltage during constantcurrent (0.5 A/cm²) SOFC testing also is shown in FIG. 16. Based onthese data, it can be concluded that this cell, which contains anode andcathode materials of the present invention, exhibits high SOFCperformance and resists degradation by sulfur. However, the sulfurresistance of the cell was not as good as the cell of Example 6 becausecobalt was not present in the current carrying anode layer.

Example 9

A solid oxide fuel cell was prepared from a 1.9 cm diameter disc-shapedFlexCell substrate made from ScSZ-6 electrolyte material with an activearea of 0.89 cm². This cell was the same as Example 8 except thatzirconia-doped ceria (ZDC) partially replaced SDC in theelectrochemically active anode layer of the bi-layer anode. This cellwas fabricated as described below:

Deposition of Interfacial Layer Coatings:

Cobalt-doped SDC interfacial layer inks were prepared and interfaciallayers were applied to both the anode and cathode faces of a FlexCellsubstrate using the same materials, methods and thermal treatments asdescribed in Example 6.

Preparation of Electrochemically Active Anode Ink.

NiO—CoO/SDC/ZDC anode precursor powders prepared as described in Example3 were used to prepare electrochemically active anode inks as follows.SDC-10 and ZDC-50 powders were prepared with different surface areas. Amixture was prepared that contained 30 grams coarse NiO—CoO/ZDC/SDCprecursor powder, 30 grams fine NiO—CoO/ZDC/SDC precursor powder, 17.5grams SDC-10 powder with a surface area of 6.0 m²/gram, 17.5 gramsZDC-50 powder with a surface area of 7.2 m²/gram, and 5 grams SDC-10powder with a surface area of 45 m²/gram. This mixture was ball milledin acetone with zirconia grinding media and the resulting slurry wasdried to a powder. A portion of this powder was dispersed into aterpineol-based ink vehicle to prepare a NiO—CoO/SDC/ZDCelectrochemically active anode ink.

Deposition of Anode Coatings.

A two-layer anode was applied to the anode face of theinterfacial-layer-coated FlexCell using the electrochemically active(NiO—CoO/ZDC/SDC) anode ink that was prepared above and the currentcarrying (NiO/ScSZ) anode ink prepared as described in Example 8. Thecoatings were deposited and thermally treated as described in Example 6.

Deposition of Bi-Layer Cathode Coatings.

Electrochemically active (LSZF/GDC/Pd) and current carrying (LSZF/Pd)cathode inks were prepared and cathode coatings were deposited onto thecobalt-doped ceria interfacial layer on the cathode face of the FlexCellsubstrate using the same materials, methods and thermal treatments asdescribed in Example 6.

The SOFC performance of this cell with hydrogen and air as fuel andoxidant (without H₂S in the fuel) was measured using button-cell testingmethods. The effect of 200 ppm H₂S on the SOFC performance (currentdensity versus power density curves) at 800° C. for this cell is shownin FIG. 16. These data confirm that very high performance is obtainedwithout H₂S in the fuel and that a relatively low degradation in SOFCperformance is observed when H₂S is present in the fuel. The effects ofvarying levels of H₂S (20-200 ppm) on cell voltage during constantcurrent (0.5 A/cm²) SOFC testing is shown in FIG. 17. Based on thesedata, it can be concluded that this cell, which contains anode andcathode materials of the present invention, exhibits high SOFCperformance and resists degradation by sulfur. However, the sulfurresistance of the cell was not as good as the cell of Example 6 (shownin FIGS. 11 and 15), because cobalt was not present in the currentcarrying anode layer. However, the sulfur resistance of this cell wasthe same as that of Example 8, which suggests that there is no detrimentto replacing at least some of the SDC in the electrochemically activeanode with ZDC.

Example 10

A solid oxide fuel cell was prepared from a 1.9 cm diameter disc-shapedFlexCell substrate made from ScSZ-6 electrolyte material with an activearea of 0.89 cm². This cell was the same as Example 8 except that thebi-layer anode was applied directly to the anode face of the FlexCellsubstrate (without an anode/electrolyte interfacial layer). This cellwas fabricated as described below:

Deposition of Cathode/Electrolyte/Interfacial Layer:

Cobalt-doped SDC interfacial layer inks were prepared and a bi-layerinterfacial layer was applied to the cathode face of a FlexCellsubstrate using the same materials, methods and thermal treatments asdescribed in Example 6.

Deposition of Anode Coatings.

A bi-layer anode was applied to the anode face of thenon-interfacial-layer-coated FlexCell substrate using electrochemicallyactive (NiO—CoO/SDC) anode ink that was prepared as described in Example6 and the current carrying (NiO/ScSZ) anode ink that was prepared asdescribed in Example 8. The coatings were deposited and thermallytreated as described in Example 6.

Deposition of Bi-Layer Cathode Coatings.

Electrochemically active (LSZF/GDC/Pd) and current carrying (LSZF/Pd)cathode inks were prepared and cathode coatings were deposited onto thecobalt-doped ceria interfacial layer on the cathode face of the FlexCellsubstrate using the same materials, methods and thermal treatments asdescribed in Example 6.

The SOFC performance of this cell with hydrogen and air as fuel andoxidant (with and without H₂S in the fuel) was measured usingbutton-cell testing methods. Data presented in FIG. 16 suggest that theperformance of this cell is not high, especially when compared to thecell of Example 8, which is identical except that the cell of Example 8was prepared with a cobalt-doped SDC interfacial layer on the anodeside. This comparison confirms the beneficial effect of the cobalt-dopedSDC interfacial layer on SOFC performance. The effects of varying levelsof H₂S (20-200 ppm) on cell voltage during constant current (0.5 A/cm²)SOFC testing is shown in FIG. 17. This cell exhibited low resistance todegradation by sulfur, especially when compared to data obtained for thecell of Example 8 (shown in FIG. 11). This comparison suggests that thecobalt-doped SDC interfacial layer is important not only for achievinghigh SOFC performance but also for achieving high resistance todegradation by sulfur.

Example 11

A solid oxide fuel cell was prepared from a 10×10 cm FlexCell substratemade from ScSZ-6 electrolyte material with an active area of 28 cm².This cell was fabricated with cobalt-doped SDC interfacial layers,electrochemically active (NiO—CoO/SDC) and current carrying (NiO/ScSZ)anode layers, and electrochemically active (LSZF/GDC/Pd) and currentcarrying (LSZF/Pd) cathode layers identical to those described inExample 6. The SOFC performance of this large-area cell was tested withhydrogen and air as fuel and oxidant (with and without 200 ppm H₂S inthe fuel) at temperatures of 750, 800 and 850° C. These data, shown inFIG. 12, confirm that the disclosed multilayer anode system provides theunprecedented combination of high SOFC performance and resistance todegradation by sulfur.

Example 12

A solid oxide fuel cell was prepared from a 10×10 cm FlexCell substratemade from ScSZ-6 electrolyte material with an active area of 28 cm².This cell was fabricated as described below:

Deposition of Interfacial Layer Coatings:

Cobalt-doped SDC interfacial layer inks were prepared and interfaciallayers were applied to both the anode and cathode faces of a FlexCellsubstrate using the same materials, methods and thermal treatments asdescribed in Example 6.

Preparation of Electrochemically Active Anode Ink.

Fine NiO—CoO anode precursor powder was made as described in Example 1.Gadolinium-doped ceria powder of the composition(Ce_(0.90)Gd_(0.10))O_(1.95) (GDC-10) was prepared with differentsurface areas. A mixture was prepared that contained 30 grams non-millednickel oxide powder (NiO, Alfa Aesar), 30 grams fine NiO—CoO precursor,35 grams GDC-10 powder with a surface area of 6.6 m²/gram, and 5 gramsGDC-10 powder with a surface area of 44 m²/gram. This mixture was ballmilled in acetone with zirconia grinding media and the resulting slurrywas dried to a powder. A portion of this powder was dispersed into aterpineol based ink vehicle to prepare a NiO—CoO/GDC electrochemicallyactive anode ink.

Deposition of Anode Coatings.

A two-layer anode was applied to the anode face of theinterfacial-layer-coated FlexCell substrate using electrochemicallyactive (NiO—CoO/GDC) anode ink prepared above and current carrying(NiO/ScSZ) anode ink as described in Example 8. The coatings weredeposited and thermally treated as described in Example 6.

Preparation of Electrochemically Active Cathode Ink.

The electrochemically active LSZF/GDC cathode ink was prepared in amanner identical to that described in Example 6 except that no palladiumwas added during preparation of the ink.

Preparation of Electrochemically Active Cathode Ink.

The current carrying LSZF cathode ink was prepared in a manner identicalto that described in Example 6, except that no palladium was addedduring preparation of the ink.

Deposition of Bi-Layer Cathode Coatings.

Electrochemically active (LSZF/GDC) and current carrying (LSZF) cathodeinks prepared above were deposited onto the cobalt-doped SDC interfaciallayer on the cathode face of the FlexCell using the same methods andthermal treatments as described in Example 6.

The SOFC performance of this large-area cell was tested with hydrogenand air as fuel and oxidant at temperatures of 750, 800 and 850° C. Asshown in FIG. 18, this cell exhibited very high power density underconditions of high fuel and air flow rates, equivalent to conventionalanode supported cells. As shown in FIG. 19, much of the performance wasretained when the cell was operated with lower fuel and air flow rateswhere fuel use was more than 90 percent. Electrical efficiencies ofapproximately 50 percent (calculated as the ratio of power output fromthe cell to the lower heating value of input fuel to cell) were obtainedwith cell power densities of approximately 0.50 W/cm² at temperatures of750 and 800° C. These data confirm that the novel electrode materialssystems of the present invention are capable of achieving highperformance even when operating under high fuel use conditions.

Example 13

A solid oxide fuel cell was prepared from a 10×10 cm HybridCellsubstrate with an active area of 28 cm². This cell was fabricated asdescribed below:

Deposition of Interfacial Layer Coating:

Cobalt-doped SDC interfacial layer inks were prepared and a bi-layerinterfacial layer was applied to the cathode face of a HybridCellsubstrate using the same materials, methods and thermal treatments asdescribed in Example 6.

Deposition of Bi-Layer Cathode Coatings.

Electrochemically active (LSZF/GDC/Pd) and current carrying (LSZF/Pd)cathode inks were prepared and cathode coatings were deposited onto thecobalt-doped ceria interfacial layer on the cathode face of theHybridCell using the same materials, methods and thermal treatments asdescribed in Example 6.

The SOFC performance of this large-area cell was tested with hydrogenand air as fuel and oxidant at a temperature of 750° C., with a constantcurrent of 0.71 A/cm² being applied and voltage being monitored for morethan a year (8800 hours). As shown in FIG. 13, there was essentially nodegradation observed in this cell after the first 150 hours. This is aremarkable result given that the cell was tested with manifolds made ofInconel-601, a high-chrome alloy, and degradation would be expected tooccur due to chromium evaporation from the manifolds. The chromiumresistance is presumed to be due to the combination of the cobalt-dopedceria interfacial layer, the use of LSZF as the ceramic electrodematerial constituent in the electrochemically active and currentcarrying cathode layers, and the use of palladium as a dopant within thecathode layers.

Example 14

A solid oxide fuel cell was prepared from a 10×10 cm FlexCell substratemade from ScSZ-6 electrolyte material with an active area of 28 cm².This cell was fabricated as described below:

Deposition of Interfacial Layer Coatings:

Cobalt-doped SDC interfacial layer inks were prepared and interfaciallayers were applied to both the anode and cathode faces of a FlexCellsubstrate using the same materials, methods and thermal treatments asdescribed in Example 6.

Deposition of Anode Coatings.

A two-layer anode was applied to the anode face of the interfacial-layercoated FlexCell using the electrochemically active (NiO—CoO/GDC) anodeink prepared as described in Example 12 and the current carrying(NiO/ScSZ) anode ink as described in Example 8. The coatings weredeposited and thermally treated as described in Example 6.

Deposition of Bi-Layer Cathode Coatings.

Electrochemically active (LSZF/GDC/Pd) and current carrying (LSZF/Pd)cathode inks were prepared and cathode coatings were deposited onto thecobalt-doped ceria interfacial layer on the cathode face of the FlexCellsubstrate using the same materials, methods and thermal treatments asdescribed in Example 6.

This large-area cell was subjected to long-term, thermal cyclingtesting. After the cell was reduced and its performance characteristicsdetermined, the cell was operated at 800° C. under steady-state,constant current conditions (0.7 A/cm²). Each thermal cycle involved thefollowing: removal of the electrical load and returning the cell to opencircuit conditions, purging the hydrogen fuel line with nitrogen,cooling to below 50° C. at a rate of 3° C./min with nitrogen flowingthrough the anode chamber and air through the cathode chamber, heatingback up to 800° C. with nitrogen flowing through the anode chamber andair through the cathode chamber, re-introducing hydrogen fuel flow andreturning the cell to open circuit conditions, and applying theelectrical load and returning the cell to its original operatingcondition (0.7 A/cm²). The cell was subjected to ten thermal cycles over570 hours of testing. As shown in FIG. 20, the cell voltage degraded bya total of 28.5 percent during the test.

Example 15

A solid oxide fuel cell was prepared from a 10×10 cm FlexCell substratemade from ScSZ-6 electrolyte material with an active area of 28 cm².This cell was fabricated as described below:

Deposition of Interfacial Layer Coatings:

Cobalt-doped SDC interfacial layer inks were prepared and interfaciallayers were applied to both the anode and cathode faces of a FlexCellsubstrate using the same materials, methods and thermal treatments asdescribed in Example 6.

Deposition of Anode Coatings.

A two-layer anode was applied to the anode face of theinterfacial-layer-coated FlexCell using the electrochemically active(NiO—CoO/SDC) anode ink prepared as described in Example 6 and thecurrent carrying (NiO/ScSZ) anode ink as described in Example 8. Thecoatings were deposited and thermally treated as described in Example 6.

Deposition of Bi-Layer Cathode Coatings.

Electrochemically active (LSZF/GDC/Pd) and current carrying (LSZF/Pd)cathode inks were prepared and cathode coatings were deposited onto thecobalt-doped ceria interfacial layer on the cathode face of the FlexCellsubstrate using the same materials, methods and thermal treatments asdescribed in Example 6.

This large-area cell was subjected to long-term, thermal cycling testingfollowing the protocol described in Example 14. The cell was subjectedto 23 thermal cycles over 1200 hours of testing. As shown in FIG. 21,the cell voltage degraded by approximately 1.7 percent during the test.The much lower thermal cycling degradation rate observed for this cellcompared to the cell of Example 14 was due to the incorporation ofnanoscale SDC within the electrochemically active anode layer of themultilayer anode system.

Example 16

A solid oxide fuel cell was prepared from a 1.9-cm diameter FlexCellsubstrate made from Gd-doped ceria (GDC-10, ten mole percent Gd) with anactive area of 0.89 cm². A two-layer anode was applied to the anode faceof the non-interfacial-layer-coated GDC FlexCell substrate usingelectrochemically active (NiO—CoO/SDC) anode ink prepared as describedin Example 6 and current carrying (NiO/ScSZ) anode ink as described inExample 8. The anode coatings were deposited and thermally treated asdescribed in Example 6. Cathode powder of the composition(Sm_(0.5)Sr_(0.5))CoO₃ (SSC) was prepared and an SSC cathode ink wasprepared by dispersion of the SSC powder into a terpineol-based inkvehicle. A single-layer SSC cathode coating was deposited on thenon-interfacial-layer-coated cathode face of the GDC FlexCell substrateThe SOFC performance of this cell with hydrogen and air as fuel andoxidant was measured using button-cell testing methods at temperaturesof 600 and 650° C. (see FIG. 22).

Example 17

A solid oxide fuel cell was prepared from a 1.9-cm diameter FlexCellsubstrate made from Gd-doped ceria (GDC-10, ten mole percent Gd) with anactive area of 0.89 cm². Cobalt-doped SDC interfacial layer inks wereprepared and interfacial layers were applied to both the anode andcathode faces of the GDC FlexCell substrate using the same materials,methods and thermal treatments as described in Example 6. A two-layeranode was applied to the anode face of the interfacial-layer-coated GDCFlexCell substrate using electrochemically active (NiO—CoO/SDC) anodeink prepared as described in Example 6 and current carrying (NiO/ScSZ)anode ink as described in Example 8. The anode coatings were depositedand thermally treated as described in Example 6. A single-layer SSCcathode coating was applied to the cathode face of FlexCell substrate asdescribed in Example 16. The SOFC performance of this cell with hydrogenand air as fuel and oxidant was measured using button-cell testingmethods at temperatures of 600 and 650° C. (see FIG. 22). As shown bythese data, the incorporation of the two-layer, cobalt-doped SDCinterfacial layer led to a significant improvement in SOFC performanceat these low operating temperatures. This improvement is presumed to bedue to an increase of triple-phase boundary area at theelectrolyte/cathode and electrolyte/anode interfaces that was providedby the bi-layer cobalt-doped SDC interfacial layer (i.e., the porous andtextured nature of the second layer of the cobalt-doped ceria bi-layerinterfacial layer).

The SOFC anode materials systems of the present invention are applicableto planar solid oxide fuel cells of electrolyte supported configurationsas well as other planar solid oxide fuel cells including anode-supportedand cathode supported types. These anode materials systems also areapplicable to non-planar solid oxide fuel cell designs including cathodesupported tubular designs, electrolyte supported tubular designs, andhybrid designs such as segmented-in-series designs. The SOFC cathodematerials systems of the present invention are applicable to planarsolid oxide fuel cells of electrolyte supported and anode supportedconfigurations, and also can be adapted for other planar solid oxidefuel cells, including cathode supported designs. These cathode materialssystems also are applicable to non-planar solid oxide fuel cell designs,such as anode supported tubular designs, electrolyte supported tubulardesigns, and hybrid designs such as segmented-in-series designs. Theelectrode materials systems of the present invention also may be adaptedto other types of electrochemical systems, including solid oxideelectrolysis systems for producing hydrogen and/or oxygen, reversiblesolid oxide fuel cell systems that cycle between power generation andreactant (hydrogen and oxygen) production, ceramic oxygen generationsystems for separating oxygen from air, and ceramic membrane reactorsfor producing hydrogen and/or other chemicals from hydrocarbonfeedstocks.

Several materials and processes are disclosed herein that allow thefabrication of planar solid oxide fuel cells with high electrochemicalperformance and exceptional long-term durability. Although many of thematerials and processes were described with reference to planar solidoxide fuel cells based on the FlexCell and HybridCell structures, thesesame materials and components have utility in other types of planarsolid oxide fuel cells and other electrochemical systems based on planarstacks of electrochemical cells. Moreover, many of the disclosedmaterial solutions can be applied to non-planar electrochemical cellconfigurations, such as tubular and flat-tubular cell designs ofcathode-supported, anode supported and electrolyte supportedconfigurations.

The preferred embodiment of this invention can be achieved by manytechniques and methods known to persons who are skilled in this field.To those skilled and knowledgeable in the arts to which the presentinvention pertains, many widely differing embodiments will be suggestedby the foregoing without departing from the intent and scope of thepresent invention. The descriptions and disclosures herein are intendedsolely for purposes of illustration and should not be construed aslimiting the scope of the present invention which is described by thefollowing claims.

What is claimed is:
 1. An anode for a solid oxide fuel cell, comprisinga cermet having a metallic component and a ceramic component, wherein:(a) the metallic component of the cermet comprises— (i) grainscontaining nickel and cobalt, and (ii) nanoscale particles of a ceramicmaterial resident within said grains; and (b) the ceramic component ofthe cermet comprises a ceramic electrolyte material having a particlesize which is larger than that of the nanoscale ceramic particles of themetallic component.
 2. The cermet anode of claim 1, wherein thenanoscale ceramic material of the metallic component is selected from adoped zirconia electrolyte material, a doped ceria electrolyte material,and combinations thereof.
 3. A solid oxide fuel cell, the solid oxidefuel cell comprising: a ceramic electrolyte membrane; a bi-layeranode/electrolyte interfacial layer on one face of the ceramicelectrolyte membrane, the anode/electrolyte interfacial layercomprising: a first thin dense ceria layer on the surface of the ceramicelectrolyte membrane; and a second thin porous ceria layer on the denseceria layer; wherein said first and second ceria layers have a differentcomposition than the electrolyte membrane an anode applied to the secondporous layer of the bi-layer anode/electrolyte interfacial layer,wherein said anode has a different composition than said bi-layeranode/electrolyte interfacial layer; and a cathode applied to theopposite face of the ceramic electrolyte membrane.
 4. The solid oxidefuel cell of claim 3, wherein the thin dense ceria layer of the bi-layeranode/electrolyte interfacial layer comprises a doped ceria electrolytematerial and the thin porous ceria layer of the bi-layeranode/electrolyte interfacial layer comprises a doped ceria electrolytematerial.
 5. The solid oxide fuel cell of claim 4, wherein the ceramicelectrolyte membrane comprises a doped zirconia electrolyte material. 6.The solid oxide fuel cell of claim 4, wherein at least one of the firstdense ceria layer and the second porous ceria layer of the bi-layeranode/electrolyte interfacial layer is doped with cobalt.
 7. A solidoxide fuel cell system capable of operating with sulfur-containingfuels, comprising: a ceramic electrolyte membrane; an anode located onone side of the electrolyte membrane; and a cathode located on theopposite side of the electrolyte membrane as the anode; wherein saidanode comprises a first cermet anode layer located adjacent to saidelectrolyte membrane, the metallic component of the first cermetcomprising at least one of an alloy containing nickel and cobalt, and amixture of nickel and cobalt compositions, and the ceramic component ofthe first cermet comprising a doped ceria electrolyte material; and asecond cermet anode layer on the first cermet anode layer, the metalliccomponent of the second cermet comprising at least one of nickel, analloy containing nickel and cobalt, and a mixture of nickel and cobaltcompositions, the ceramic component of the second cermet comprising aceramic electrolyte material, and the cermet having a coarsermicrostructure and a higher metal content than the first cermet layer;wherein said first and second cermet anode layers comprise 20 to 80percent by weight of ceramic component, and said anode layers arearranged such that the first cermet anode layer is located closer to theelectrolyte membrane than the second anode layer is.
 8. The solid oxidefuel cell system of claim 7, wherein the ceramic electrolyte material inthe second cermet layer is selected from a doped zirconia electrolytematerial, a doped ceria electrolyte material, and combinations thereof.9. The solid oxide fuel cell system of claim 7, further comprisingnanoscale particles of a ceramic material resident within the grains ofthe metallic component of at least one of the cermet layers, thenanoscale ceramic material being selected from a doped ceria electrolytematerial, a doped zirconia electrolyte material, and combinationsthereof.
 10. The solid oxide fuel cell system of claim 7, wherein themetallic component of at least one of the cermet anode layers comprisesan alloy of nickel and cobalt.
 11. The anode of claim 1, wherein thenanoscale ceramic material of the metallic component comprises a dopedceria electrolyte material.
 12. A solid oxide fuel cell system capableof operating with sulfur-containing fuels, comprising: a ceramicelectrolyte membrane; an anode located on one side of the electrolytemembrane; and a cathode located on the opposite side of the electrolytemembrane as the anode; wherein said anode comprises the anode ofclaim
 1. 13. A solid oxide fuel cell system capable of operating withsulfur-containing fuels, comprising: a ceramic electrolyte membrane; ananode located on one side of the electrolyte membrane; and a cathodelocated on the opposite side of the electrolyte membrane as the anode;wherein said anode comprises the anode of claim
 11. 14. The solid oxidefuel cell of claim 4, wherein both the thin dense ceria layer and thethin porous ceria layer of the bi-layer anode/electrolyte interfaciallayer comprise samarium-doped ceria.
 15. The solid oxide fuel cell ofclaim 6, wherein both the first dense ceria layer and the second porousceria layer of the bi-layer anode/electrolyte interfacial layer aredoped with cobalt.